Input-Output Device and Method for Driving the Same

ABSTRACT

To increase the light detection accuracy, a method for driving an input-output device that includes a first light unit, a second light unit, a display circuit, and Y light detection circuits (Y is a natural number of 2 or more) is proposed. The same light-detection control signal is input to the Y light detection circuits. In a frame period set by a display selection signal, the first light unit is lit by sequentially switching Z light-emitting diodes and emitting light, and the second light unit is lit by making the white light-emitting diode emit light when the first light unit is not lit. In a period when the second light unit is lit, Y pieces of data corresponding to the illuminance of light entering the Y light detection circuits are generated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to an input-outputdevice. Another embodiment of the present invention relates to a methodfor driving an input-output device.

2. Description of the Related Art

In recent years, devices having a function of outputting data and afunction of inputting data with incident light (such devices are alsoreferred to as input-output devices) have been developed.

An example of an input-output device is an input-output device thatincludes a pixel portion including a plurality of light detectioncircuits (also referred to as optical sensors) arranged in the row andcolumn directions and, as a light source, a backlight includinglight-emitting diodes of a plurality of colors (e.g., Patent Document1). In the input-output device disclosed in Patent Document 1, in eachframe period, the backlight is lit while colors of emitted light areswitched to display a full-color image, and light reflected by an objectsubjected to reading is read as data. That is, the input-output devicedisclosed in Patent Document 1 functions as a touch panel. Note that amethod in which a backlight is lit while colors of emitted light areswitched in each frame period is called a field sequential method.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. H11-008741

SUMMARY OF THE INVENTION

A conventional input-output device has a problem of low light detectionaccuracy.

For example, a conventional input-output device employs a rollingshutter method in which light detection circuits in each row generateand output data corresponding to the illuminance of incident light (suchdata is referred to as optical data). In the case where the conventionalinput-output device employs a field sequential method, a plurality oflight-emitting diodes need to be sequentially switched and emit light inone frame period so as to switch lighting states of the backlight.Therefore, in order to generate optical data corresponding to eachlighting state of the backlight, the light detection circuits in eachrow need to generate optical data so that optical data is generated inall the light detection circuits in a period during which the backlightis lit. Consequently, a time in which light enters each light detectioncircuit at the time of generating optical data is short, so that thelight detection accuracy is decreased.

In addition, for example, light in the environment where theinput-output device is positioned, such as external light, enters theinput-output device. Such environmental light causes noise when opticaldata is generated, so that the light detection accuracy is reduced. Forexample, in the case where data is input to the input-output device whenlight reflected by a finger enters the light detection circuit as in atouch panel, environmental light sometimes causes the light reflected bythe finger and light reflected by a hand portion other than the fingerto be recognized as equivalent data.

An object of one embodiment of the present invention is to increase thelight detection accuracy.

According to one embodiment of the present invention, an input-outputdevice includes a display circuit, a plurality of light detectioncircuits, a first light unit including a plurality of firstlight-emitting diodes, and a second light unit including a secondlight-emitting diode and a light guide plate. Light from the secondlight-emitting diode enters the light guide plate. The first light unitis lit by sequentially switching the plurality of first light-emittingdiodes and emitting light every unit period. The second light unit islit by making the second light-emitting diode emit light when the firstlight unit is not lit. The plurality of light detection circuitsgenerate optical data in accordance with the same signal when the secondlight unit is lit in the unit period. Accordingly, adverse effects oflight in the environment where the input-output device is positioned aresuppressed.

One embodiment of the present invention is an input-output device thatincludes a first light unit including Z light-emitting diodes (Z is anatural number of 3 or more); a second light unit including a whitelight-emitting diode and a light guide plate on which light from thewhite light-emitting diode is incident; an X display circuit (X is anatural number) provided between the first light unit and the secondlight unit, supplied with a display selection signal, supplied with adisplay data signal in accordance with the display selection signal, andset in a display state corresponding to data of the inputted displaydata signal; and Y light detection circuits (Y is a natural number of 2or more) provided between the first light unit and the second lightunit, supplied with the same light-detection control signal, andconfigured to generate data corresponding to illuminance of incidentlight in accordance with the inputted light-detection control signal.

One embodiment of the present invention is a method for driving aninput-output device that includes a first light unit including Zlight-emitting diodes (Z is a natural number of 3 or more); a secondlight unit including a white light-emitting diode and a light guideplate on which light from the white light-emitting diode is incident; anX display circuit (X is a natural number) provided between the firstlight unit and the second light unit, supplied with a display selectionsignal, supplied with a display data signal in accordance with thedisplay selection signal, and set in a display state corresponding todata of the inputted display data signal; and Y light detection circuits(Y is a natural number of 2 or more) provided between the first lightunit and the second light unit, supplied with a light-detection controlsignal, and configured to generate data corresponding to illuminance ofincident light in accordance with the inputted light-detection controlsignal. The same light-detection control signal is input to the Y lightdetection circuits. In a frame period set by the display selectionsignal, the first light unit is lit by sequentially switching the Zlight-emitting diodes and emitting light, and the second light unit islit by making the white light-emitting diode emit light when the firstlight unit is not lit. Y pieces of data corresponding to illuminance oflight incident on the Y light detection circuits are generated in aperiod when the second light unit is lit.

Another embodiment of the present invention is a method for driving aninput-output device that includes a first light unit including Zlight-emitting diodes (Z is a natural number of 3 or more); a secondlight unit including a white light-emitting diode and a light guideplate on which light from the white light-emitting diode is incident; anX display circuit (X is a natural number) provided between the firstlight unit and the second light unit, supplied with a display selectionsignal, supplied with a display data signal in accordance with thedisplay selection signal, and set in a display state corresponding todata of the inputted display data signal; and Y light detection circuits(Y is a natural number of 2 or more) provided between the first lightunit and the second light unit, supplied with a light-detection controlsignal, and configured to generate data corresponding to illuminance ofincident light in accordance with the inputted light-detection controlsignal. The same light-detection control signal is input to the Y lightdetection circuits. In a frame period set by the display selectionsignal, the first light unit is lit by sequentially switching the Zlight-emitting diodes and emitting light, and the second light unit islit by making the white light-emitting diode emit light when the firstlight unit is not lit. Y pieces of first data corresponding toilluminance of light incident on the Y light detection circuits aregenerated in a period when the second light unit is lit, and Y pieces ofsecond data corresponding to illuminance of light incident on the Ylight detection circuits are generated in a period when the first lightunit and the second light unit are not lit. Third data that is data ofdifference between the first data and the second data is generated.

According to one embodiment of the present invention, the lightdetection accuracy can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate an example of an input-output device inEmbodiment 1;

FIGS. 2A and 2B illustrate an example of an input-output device inEmbodiment 2;

FIGS. 3A to 3D illustrate examples of a light detection circuit inEmbodiment 3;

FIGS. 4A to 4D illustrate examples of a display circuit in Embodiment 4;

FIG. 5 is a schematic cross-sectional view illustrating a structuralexample of a light unit in Embodiment 5;

FIGS. 6A to 6D are schematic cross-sectional views each illustrating astructural example of a transistor in Embodiment 6;

FIGS. 7A to 7E are schematic cross-sectional views illustrating anexample of a process for manufacturing the transistor illustrated inFIG. 6A;

FIGS. 8A and 8B are diagrams for explaining a circuit for evaluatingcharacteristics;

FIG. 9A is a graph showing a relation between elapsed time Time ofmeasurement of Sample 4 (SMP4), Sample 5 (SMP5), and Sample 6 (SMP6) andoutput voltage Vout, and FIG. 9B is a graph showing a relation betweenelapsed time Time and leakage current calculated by the measurement;

FIG. 10 is a graph showing a relation between voltage of a node A andleakage current estimated by measurement;

FIG. 11 is a graph showing a relation between voltage of a node A andleakage current estimated by measurement;

FIG. 12 is a graph showing a relation between voltage of a node A andleakage current estimated by measurement;

FIG. 13 is a graph showing a relation between voltage of a node A andleakage current estimated by measurement;

FIGS. 14A and 14B illustrate a structural example of an active matrixsubstrate in Embodiment 7;

FIGS. 15A and 15B illustrate a structural example of an active matrixsubstrate in Embodiment 7;

FIGS. 16A and 16B each illustrate a structural example of aninput-output device in Embodiment 7; and

FIGS. 17A to 17F each illustrate a structural example of an electronicdevice in Embodiment 8.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments for explaining the present invention will bedescribed below with reference to the accompanying drawings. Note thatthe present invention is not limited to the description below, and it iseasily understood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention should not beinterpreted as being limited to the following description of theembodiments.

Note that the contents in different embodiments can be combined witheach other as appropriate. In addition, the contents in differentembodiments can be replaced with each other.

Embodiment 1

In this embodiment, an input-output device that can output data and caninput data with incident light will be described.

An example of an input-output device in this embodiment will bedescribed with reference to FIGS. 1A to 1C. FIGS. 1A to 1C are diagramsfor explaining an example of the input-output device in this embodiment.

First, a structural example of the input-output device in thisembodiment will be described with reference to FIG. 1A. FIG. 1A is aschematic diagram illustrating the structural example of theinput-output device in this embodiment.

The input-output device illustrated in FIG. 1A includes a displayselection signal output circuit (DSELOUT) 101, a display data signaloutput circuit (DDOUT) 102, a light-detection reset signal outputcircuit (PRSTOUT) 103 a, a light-detection control signal output circuit(PCTLOUT) 103 b, an output selection signal output circuit (OSELOUT) 103c, a light unit (LIGHT) 104 a, a light unit 104 b, an X display circuit(DISP, X is a natural number) 105 d, Y light detection circuits (PS, Yis a natural number of 2 or more) 105 p, and a read circuit (READ) 106.

The display selection signal output circuit 101 has a function ofoutputting a plurality of display selection signals (signals DSEL) whichare pulse signals.

The display selection signal output circuit 101 includes a shiftregister, for example. The display selection signal output circuit 101can output a display selection signal by output of a pulse signal fromthe shift register.

An image signal which is an electric signal for displaying an image isinput to the display data signal output circuit 102. The display datasignal output circuit 102 has a function of generating a display datasignal (a signal DD) which is a voltage signal on the basis of theinputted image signal and outputting the generated display data signal.

The display data signal output circuit 102 includes a transistor, forexample.

In the input-output device, the transistor has two terminals and acurrent control terminal that controls a current flowing between the twoterminals with an applied voltage. Note that without limitation to thetransistor, terminals where a current flowing therebetween is controlledare referred to as current terminals. Two current terminals are alsoreferred to as a first current terminal and a second current terminal.

In the input-output device, the transistor can be a field-effecttransistor, for example. In a field-effect transistor, a first currentterminal is one of a source and a drain, a second current terminal isthe other of the source and the drain, and a current control terminal isa gate.

Voltage generally refers to a difference between potentials at twopoints (also referred to as a potential difference). However, values ofboth a voltage and a potential are sometimes expressed in volts (V) in acircuit diagram or the like, so that it is difficult to distinguishbetween them. Therefore, in this specification, a potential differencebetween a potential at one point and a potential to be the reference(also referred to as a reference potential) is used as a voltage at thepoint in some cases.

The display data signal output circuit 102 can output data of an imagesignal as a display data signal when the transistor is on. Thetransistor can be controlled by input of a control signal which is apulse signal to the current control terminal. In the case where thereare a plurality of display circuits 105 d, the display data signaloutput circuit 102 may output data of an image signal as a plurality ofdisplay data signals by selectively turning on or off a plurality oftransistors.

The light-detection reset signal output circuit 103 a has a function ofoutputting a light-detection reset signal (a signal PRST) which is apulse signal.

The light-detection reset signal output circuit 103 a includes a shiftregister, for example. The light-detection reset signal output circuit103 a can output a light-detection reset signal by output of a pulsesignal from the shift register.

The light-detection control signal output circuit 103 b has a functionof outputting a light-detection control signal (a signal PCTL) which isa pulse signal.

The light-detection control signal output circuit 103 b includes a shiftregister, for example. The light-detection control signal output circuit103 b can output a light-detection control signal by output of a pulsesignal from the shift register.

The output selection signal output circuit 103 c has a function ofoutputting an output selection signal (a signal OSEL) which is a pulsesignal.

The output selection signal output circuit 103 c includes a shiftregister, for example. The output selection signal output circuit 103 ccan output an output selection signal by output of a pulse signal fromthe shift register.

Each of the light units 104 a and 104 b is a light-emitting unitincluding a light source.

The light unit 104 a includes Z light-emitting diodes (LEDs) A (Z is anatural number of 3 or more) as light sources. The Z light-emittingdiodes A are light-emitting diodes that emit light with a wavelength inthe visible light region (e.g., a region with a wavelength of 360 nm to830 nm). As the Z light-emitting diodes A, a red light-emitting diode, agreen light-emitting diode, and a blue light-emitting diode can be used,for example. Note that the number of light-emitting diodes of differentcolors may be more than one. Alternatively, as the Z light-emittingdiodes A, a light-emitting diode of another color (e.g., a whitelight-emitting diode) may be used in addition to the red, green, andblue light-emitting diodes.

For example, light emission of the light-emitting diodes A may becontrolled using a control signal that selects a light-emitting diode Ato which a voltage is applied. Further, the light unit 104 a may beprovided with a light control circuit for outputting a control signalthat controls whether to select a light-emitting diode A to which avoltage is applied.

The light unit 104 b includes a light-emitting diode B as a light sourceand a light guide plate. The light-emitting diode B emits light with awavelength in the visible light region. As the light-emitting diode, awhite light-emitting diode can be used, for example. Note that thenumber of white light-emitting diodes may be more than one. Light fromthe light-emitting diode B enters the light guide plate.

For example, when an object to be read is in contact with the lightguide plate while the light unit 104 b having the above structure is on,light from the light source is scattered at the contact portion of theobject and the light guide plate and enters the light detection circuit105 p.

For example, light emission of the light-emitting diode B may becontrolled using a control signal that selects a light-emitting diode Bto which a voltage is applied. Further, the light unit 1046 may beprovided with a light control circuit for outputting a control signalthat controls whether to select a light-emitting diode B to which avoltage is applied.

The display circuit 105 d is provided between the light unit 104 a andthe light unit 104 b. To the display circuit 105 d, a display selectionsignal which is a pulse signal is input, and a display data signal isinput in accordance with the inputted display selection signal. Thedisplay circuit 105 d changes its display state in accordance with dataof the inputted display data signal.

The display circuit 105 d includes a display selection transistor and adisplay element, for example.

The display selection transistor has a function of selecting whetherdata of a display data signal is input to the display element.

The display element changes its display state corresponding to data of adisplay data signal by input of the data of the display data signal withthe display selection transistor.

As the display element, a liquid crystal element can be used, forexample.

Examples of a display method of the input-output device including aliquid crystal element are a TN (twisted nematic) mode, an IPS (in-planeswitching) mode, a STN (super twisted nematic) mode, a VA (verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optically compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,an MVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASV (advanced super view) mode, and a FFS (fringefield switching) mode.

The light detection circuit 105 p is provided between the light unit 104a and the light unit 104 b. A light-detection reset signal, alight-detection control signal, and an output selection signal are inputto the light detection circuit 105 p.

The light detection circuit 105 p is reset in accordance with thelight-detection reset signal.

In addition, the light detection circuit 105 p has a function ofgenerating data corresponding to the illuminance of incident light (suchdata is also referred to as optical data) in accordance with thelight-detection control signal.

The light detection circuit 105 p also has a function of outputting thegenerated optical data as an optical data signal in accordance with theoutput selection signal.

The light detection circuit 105 p includes, for example, a photoelectricconversion element (PCE), a light-detection reset selection transistor,a light-detection control transistor, an amplification transistor, andan output selection transistor.

When light enters the photoelectric conversion element, a current (alsoreferred to as a photocurrent) flows through the photoelectricconversion element in accordance with the illuminance of incident light.

A current control terminal of the light-detection reset selectiontransistor is supplied with a light-detection reset signal. Thelight-detection reset selection transistor has a function of selectingwhether the voltage of a current control terminal of the amplificationtransistor is set to a reference value.

A current control terminal of the light-detection control transistor issupplied with a light-detection control signal. The light-detectioncontrol transistor has a function of controlling whether the voltage ofthe current control terminal of the amplification transistor is set to avalue corresponding to the photocurrent flowing through thephotoelectric conversion element.

A current control terminal of the output selection transistor issupplied with an output selection signal. The output selectiontransistor has a function of selecting whether optical data is output asan optical data signal from the light detection circuit 105 p.

The light detection circuit 105 p outputs optical data as an opticaldata signal from a first current terminal or a second current terminalof the amplification transistor.

The display circuit 105 d and the light detection circuit 105 p areprovided in a pixel portion 105. The pixel portion 105 is a region inwhich data is displayed and read. A pixel includes at least one displaycircuit 105 d. The pixel may further include at least one lightdetection circuit 105 p. When there are a plurality of display circuits105 d, the display circuits 105 d may be arranged in the row and columndirections in the pixel portion 105, for example. Furthermore, whenthere are a plurality of light detection circuits 105 p, the lightdetection circuits 105 p may be arranged in the row and columndirections in the pixel portion 105, for example.

The read circuit 106 has a function of selecting a light detectioncircuit 105 p from which optical data is to be read and reading opticaldata from the selected light detection circuit 105 p.

The read circuit 106 is formed using, for example, a selection circuit.For example, the selection circuit includes a transistor. The selectioncircuit can read optical data by input of an optical data signal fromthe light detection circuit 105 p with the transistor, for example.

Next, as an example of a method for driving the input-output device inthis embodiment, an example of a method for driving the input-outputdevice illustrated in FIG. 1A will be described with reference to FIGS.1B and 1C. FIGS. 1B and 1C are timing charts each illustrating anexample of a method for driving the input-output device in FIG. 1A.

In the example of a method for driving the input-output device in FIG.1A, the same light-detection control signal is input to the Y lightdetection circuits 105 p. In frame periods (e.g., frame periods f1 to fnillustrated in FIGS. 1B and 1C) set in accordance with a displayselection signal, the Z light-emitting diodes in the light unit 104 aare sequentially switched and emit light so that the light unit 104 asequentially switches its lighting state from a lighting state C1 (astate where the first light-emitting diode A emits light) to a lightingstate Ck (a state where the Z-th light-emitting diode A emits light). InFIGS. 1B and 1C, the light unit 104 a is off in an interval between twosuccessive periods during which the light unit 104 a is on.

A display data signal is input to the display circuit 105 d inaccordance with the display selection signal. When the light unit 104 ais on, the display circuit 105 d is put in a display state correspondingto data of the display data signal. For example, when the light unit 104a is in the lighting state C1, the display circuit 105 d is put in adisplay state dc1 (a display state corresponding to the lighting stateC1). When the light unit 104 a is in the lighting state C2, the displaycircuit 105 d is put in a display state dc2 (a display statecorresponding to the lighting state C2). When the light unit 104 a is inthe lighting state Ck, the display circuit 105 d is put in a displaystate dck (a display state corresponding to the lighting state C/c).

Further, when the light unit 104 a is off, the light-emitting diode B ismade to emit light and the light unit 104 b is brought into a lightingstate LT. Note that the cycle of switching the state of the light unit104 b from a non-lighting state to a lighting state is preferably longerthan 0 seconds and shorter than or equal to 1/60 seconds. Thus, flickersof display images due to blinking of the light unit 104 b can besuppressed.

When the light unit 104 b is in the lighting state LT, a pulse (shown aspls) of the light-detection control signal is input to the Y lightdetection circuits 105 p. At this time, each of the Y light detectioncircuits 105 p generates optical data. Note that when the cycle of thepulse of the light-detection control signal is made longer than thecycle of switching the lighting state of the light unit 104 a, a periodduring which optical data is not output when the light unit 104 a is offcan be provided, for example. Consequently, a period during which thelight unit 104 a is off can be efficiently used at the time ofgenerating optical data; thus, a period during which light enters thelight detection circuit at the time of generating optical data can beset longer. In addition, optical data can be output regardless of thelighting state of the light unit, so that the operation frequency of thelight detection circuit can be lowered and power consumption can bereduced.

Further, the Y light detection circuits 105 p output the generatedoptical data as optical data signals to the read circuit 106 inaccordance with output selection signals, whereby the optical data isread.

Note that the timing of when the light unit 104 b is lit may be the sameor different in frame periods.

For example, in the timing chart in FIG. 1B, in each frame period,optical data is generated by the light detection circuit 105 p in aperiod when the light unit 104 a is in a non-lighting state between thelighting state C1 and the lighting state C2.

In addition, for example, in the timing chart in FIG. 1C, optical datais generated by the light detection circuit 105 p in a different periodwhen the light unit 104 a is in a non-lighting state, depending on frameperiods.

As described with FIGS. 1A to 1C, the input-output device exemplified inthis embodiment includes a display circuit, a plurality of lightdetection circuits, a first light unit, and a second light unitincluding a light guide plate. Light from a light source enters thelight guide plate. With the above structure, light reflected by anobject to be read can enter the light detection circuit only when theobject is in contact with the light guide plate in the second lightunit; thus, the light detection accuracy can be improved.

In addition, in the input-output device exemplified in this embodiment,the same light-detection control signal is input to the Y lightdetection circuits. With the above structure, a time necessary for allthe light detection circuits to generate optical data can be madeshorter, and a period during which light enters the light detectioncircuit at the time of generating optical data can be set longer.Further, the operation frequency of the light detection circuit can belowered, and power consumption can be reduced. Note that a method wherethe same light-detection control signal is input to a plurality of lightdetection circuits is called a global shutter method.

Furthermore, in the input-output device exemplified in this embodiment,the first light unit is lit while a plurality of light-emitting diodesare sequentially switched and emit light. With the above structure, theinput-output device can display full-color images.

Further, in the input-output device exemplified in this embodiment, byinputting the same light-detection control signal to the Y lightdetection circuits, the second light unit is on when the first lightunit is off, and optical data can be generated in a plurality of lightdetection circuits when the second light unit is on. With the abovestructure, the influence of the first and second light units on displayimages can be reduced. Moreover, the influence of display images onoptical data can be reduced.

The light detection accuracy can be therefore improved with theabove-described structure.

Embodiment 2

In this embodiment, another example of the input-output device inEmbodiment 1 will be described. Note that the description of Embodiment1 is employed as appropriate for the same portions as those inEmbodiment 1.

An example of an input-output device in this embodiment will bedescribed with reference to FIGS. 2A and 2B. FIGS. 2A and 2B arediagrams for explaining an example of the input-output device in thisembodiment.

First, a structural example of the input-output device in thisembodiment will be described with reference to FIG. 2A. FIG. 2A is aschematic diagram illustrating the structural example of theinput-output device in this embodiment.

The input-output device illustrated in FIG. 2A includes a displayselection signal output circuit 101, a display data signal outputcircuit 102, a light-detection reset signal output circuit 103 a, alight-detection control signal output circuit 103 b, an output selectionsignal output circuit 103 c, a light unit 104 a, a light unit 104 b, anX display circuit 105 d, Y light detection circuits 105 p, a readcircuit 106, and a data processing circuit (DataP) 107.

The display selection signal output circuit 101, the display data signaloutput circuit 102, the light-detection reset signal output circuit 103a, the light-detection control signal output circuit 103 b, the outputselection signal output circuit 103 c, the light unit 104 a, the lightunit 104 b, the display circuit 105 d, the light detection circuit 105p, and the read circuit 106 are the same as those in the input-outputdevice illustrated in FIG. 1A; therefore, the description of thecomponents in the input-output device in FIG. 1A is employed asappropriate.

The data processing circuit 107 performs arithmetic processing on dataof an inputted data signal. The data processing circuit 107 includes amemory circuit and an arithmetic circuit. The memory circuit has afunction of storing data of the data signal. The arithmetic circuit hasa function of generating data of difference between data in a pluralityof data signals by arithmetic processing.

Note that the data processing circuit 107 may be included in theinput-output device; alternatively, a separate data processing meanshaving a function equivalent to that of the data processing circuit(e.g., a personal computer) may be electrically connected to theinput-output device. When the data processing circuit 107 is provided inthe input-output device, the number of wirings in a portion where thedata processing circuit 107 and the read circuit 106 are connected toeach other can be reduced, for example.

Next, as an example of a method for driving the input-output device inthis embodiment, an example of a method for driving the input-outputdevice illustrated in FIG. 2A will be described with reference to FIG.2B. FIG. 2B is a timing chart illustrating an example of a method fordriving the input-output device in FIG. 2A.

In the example of a method for driving the input-output device in FIG.2A, the same light-detection control signal is input to the Y lightdetection circuits 105 p. In frame periods (e.g., frame periods f1 to fnillustrated in FIG. 2B) set in accordance with a display selectionsignal, the first to Z-th light-emitting diodes A in the light unit 104a are sequentially switched and emit light so that the light unit 104 asequentially switches its lighting state from the lighting state C1 (thestate where the first light-emitting diode A emits light) to thelighting state Ck (the state where the Z-th light-emitting diode A emitslight) as illustrated in FIG. 2B. Further, the light unit 104 a is offbetween the lighting states.

A display data signal is input to the display circuit 105 d inaccordance with the display selection signal. When the light unit 104 ais on, the display circuit 105 d is put in a display state correspondingto data of the display data signal. For example, when the light unit 104a is in the lighting state C1, the display circuit 105 d is put in adisplay state dc1 (a display state corresponding to the lighting stateC1). When the light unit 104 a is in the lighting state C2, the displaycircuit 105 d is put in a display state dc2 (a display statecorresponding to the lighting state C2). When the light unit 104 a is inthe lighting state Ck, the display circuit 105 d is put in a displaystate dck (a display state corresponding to the lighting state Ck).

Further, when the light unit 104 a is off, a light-emitting diode B ismade to emit light and the light unit 104 b is brought into a lightingstate LT.

When the light unit 104 b is on, a pulse of the light-detection controlsignal is input to the Y light detection circuits 105 p. At this time,each of the Y light detection circuits 105 p generates optical data.

Further, the Y light detection circuits 105 p output the generatedoptical data as optical data signals to the read circuit 106 inaccordance with output selection signals, whereby the optical data isread. The read optical data is stored in the memory circuit in the dataprocessing circuit 107.

In addition, when the light units 104 a and 104 b are off in a frameperiod (the frame period fn in FIG. 2B) that is different from the aboveframe period, a pulse of the light-detection control signal is input tothe Y light detection circuits 105 p. At this time, each of the Y lightdetection circuits 105 p generates optical data.

Further, the Y light detection circuits 105 p output the generatedoptical data as optical data signals to the read circuit 106 inaccordance with output selection signals, whereby the optical data isread. The read optical data is stored in the memory circuit in the dataprocessing circuit 107.

Then, the arithmetic circuit in the data processing circuit 107generates data of difference between the optical data generated when thelight unit 104 b is on and the optical data generated when the lightunits 104 a and 104 b are off. The data of difference is used as datafor executing predetermined processing.

As described with FIGS. 2A and 2B, the input-output device exemplifiedin this embodiment has the structure shown in Embodiment 1, generatesoptical data at the time when the second light unit is on and opticaldata at the time when the first and second light units are off, andgenerates data of difference between these two optical data signals. Bygenerating data of difference, data of light in the environment wherethe input-output device is positioned can be removed from the opticaldata while the advantageous effects described in Embodiment 1 areobtained; thus, the light detection accuracy can be further improved.

Embodiment 3

In this embodiment, an example of a light detection circuit in theinput-output device described in Embodiments 1 and 2 will be described.

Examples of the light detection circuit in this embodiment will bedescribed with reference to FIGS. 3A to 3D. FIGS. 3A to 3D are diagramsfor explaining an example of the light detection circuit in thisembodiment.

First, configuration examples of the light detection circuit in thisembodiment will be described with reference to FIGS. 3A and 3B. FIGS. 3Aand 3B each illustrate the configuration example of the light detectioncircuit in this embodiment.

The light detection circuit illustrated in FIG. 3A includes aphotoelectric conversion element 131 a, a transistor 132 a, a transistor133 a, and a transistor 134 a.

In the light detection circuit in FIG. 3A, the transistors 132 a, 133 a,and 134 a are field-effect transistors.

The photoelectric conversion element 131 a has a first current terminaland a second current terminal. A reset signal is input to the firstcurrent terminal of the photoelectric conversion element 131 a.

One of a source and a drain of the transistor 134 a is electricallyconnected to the second current terminal of the photoelectric conversionelement 131 a. A gate of the transistor 134 a is supplied with alight-detection control signal.

A gate of the transistor 132 a is electrically connected to the other ofthe source and the drain of the transistor 134 a.

One of a source and a drain of the transistor 133 a is electricallyconnected to one of a source and a drain of the transistor 132 a. A gateof the transistor 133 a is supplied with an output selection signal.

Either the other of the source and drain of the transistor 132 a or theother of the source and drain of the transistor 133 a is supplied with avoltage Va.

The light detection circuit in FIG. 3A outputs optical data from therest of the other of the source and drain of the transistor 132 a or theother of the source and drain of the transistor 133 a, as an opticaldata signal.

The light detection circuit illustrated in FIG. 3B includes aphotoelectric conversion element 131 b, a transistor 132 b, a transistor133 b, a transistor 134 b, and a transistor 135.

In the light detection circuit in FIG. 3B, the transistors 132 b, 133 b,134 b, and 135 are field-effect transistors.

The photoelectric conversion element 131 b has a first current terminaland a second current terminal. A voltage Vb is input to the firstcurrent terminal of the photoelectric conversion element 131 b.

Note that one of the voltage Va and the voltage Vb is a high powersupply voltage Vdd, and the other thereof is a low power supply voltageVss. The high power supply voltage Vdd is relatively higher than the lowpower supply voltage Vss. The low power supply voltage Vss is relativelylower than the high power supply voltage Vdd. The values of the voltageVa and the voltage Vb are sometimes interchanged depending on thepolarity of the transistors, for example. The difference between thevoltage Va and the voltage Vb is a power supply voltage.

One of a source and a drain of the transistor 134 b is electricallyconnected to the second current terminal of the photoelectric conversionelement 131 b. A gate of the transistor 1346 is supplied with alight-detection control signal.

A gate of the transistor 132 b is electrically connected to the other ofthe source and the drain of the transistor 134 b.

A light-detection reset signal is input to a gate of the transistor 135.The voltage Va is input to one of a source and a drain of the transistor135. The other of the source and the drain of the transistor 135 iselectrically connected to the other of the source and the drain of thetransistor 134 b.

An output selection signal is input to a gate of the transistor 133 b.One of a source and a drain of the transistor 133 b is electricallyconnected to one of a source and a drain of the transistor 132 b.

The voltage Va is input to either the other of the source and drain ofthe transistor 1326 or the other of the source and drain of thetransistor 133 b.

The light detection circuit in FIG. 3B outputs optical data from therest of the other of the source and drain of the transistor 132 b or theother of the source and drain of the transistor 133 b, as an opticaldata signal.

Next, the components of the light detection circuits illustrated inFIGS. 3A and 3B will be described.

As the photoelectric conversion elements 131 a and 131 b, photodiodes orphototransistors can be used, for example. When the photoelectricconversion elements 131 a and 131 b are photodiodes, one of an anode anda cathode of the photodiode corresponds to the first current terminal ofthe photoelectric conversion element, and the other of the anode and thecathode of the photodiode corresponds to the second current terminal ofthe photoelectric conversion element. When the photoelectric conversionelements 131 a and 131 b are phototransistors, one of a source and adrain of the phototransistor corresponds to the first current terminalof the photoelectric conversion element, and the other of the source andthe drain of the phototransistor corresponds to the second currentterminal of the photoelectric conversion element.

The transistors 132 a and 132 b each serve as an amplificationtransistor.

The transistors 134 a and 134 b each serve as a light-detection controltransistor.

The transistor 135 serves as a light-detection reset selectiontransistor. Note that the transistor 135 is not necessarily provided inthe light detection circuit in this embodiment; in the case where thetransistor 135 is provided, the gate voltage of the transistor 132 b canbe reset to a desired voltage.

The transistors 133 a and 133 b each serve as an output selectiontransistor.

Examples of the transistors 132 a, 132 b, 133 a, 133 b, 134 a, 134 b,and 135 are a transistor including a semiconductor layer containing asemiconductor that belongs to Group 14 of the periodic table (e.g.,silicon) and a transistor including an oxide semiconductor layer; achannel is formed in the semiconductor layer or the oxide semiconductorlayer. For example, the use of the transistor including an oxidesemiconductor layer can suppress variation in the gate voltage due toleakage current of the transistor 132 a, 132 b, 133 a, 133 b, 134 a, 134b, or 135.

Next, examples of methods for driving the light detection circuits inFIGS. 3A and 3B will be described.

First, an example of a method for driving the light detection circuit inFIG. 3A will be described with reference to FIG. 3C. FIG. 3C is a timingchart for explaining the example of the method for driving the lightdetection circuit in FIG. 3A and shows the states of the light-detectionreset signal, the output selection signal, the photoelectric conversionelement 131 a, the transistor 133 a, and the transistor 134 a. Here, thecase where the photoelectric conversion element 131 a is a photodiode isdescribed as an example.

In the example of the method for driving the light detection circuit inFIG. 3A, first, a pulse of the light-detection reset signal is input ina period T31. Moreover, a pulse of the light-detection control signal isinput in the period T31 and a period T32. Note that in the period T31,the timing of starting input of the pulse of the light-detection resetsignal may be earlier than the timing of starting input of the pulse ofthe light-detection control signal.

At this time, in the period T31, the photoelectric conversion element131 a is set in a state where current flows in the forward direction(also referred to as a state ST51), the transistor 134 a is turned on,and the transistor 133 a is turned off.

At that time, the gate voltage of the transistor 132 a is reset to agiven value.

Next, in the period T32 after the input of the pulse of thelight-detection reset signal, the photoelectric conversion element 131 ais set in a state where voltage is applied in the reverse direction(also referred to as a state ST52), and the transistor 133 a remainsoff.

At this time, a photocurrent flows between the first current terminaland the second current terminal of the photoelectric conversion element131 a in accordance with the illuminance of light entering thephotoelectric conversion element 131 a. Further, the level of the gatevoltage of the transistor 132 a varies in accordance with thephotocurrent. At this time, the value of the channel resistance betweenthe source and the drain of the transistor 132 a is changed.

Then, in a period T33 after the input of the pulse of thelight-detection control signal, the transistor 134 a is turned off.

At this time, the gate voltage of the transistor 132 a is kept at avalue corresponding to the photocurrent of the photoelectric conversionelement 131 a in the period T32. Note that the period T33 is notnecessarily provided; however, in the case where there is the periodT33, the timing of outputting an optical data signal in the lightdetection circuit can be set as appropriate. For example, the timing ofoutputting an optical data signal can be set as appropriate in aplurality of light detection circuits.

Next, in a period T34, a pulse of the output selection signal is input.

At this time, the photoelectric conversion element 131 a remains in thestate ST52, the transistor 133 a is turned on, and a current flowsthrough the source and drain of the transistor 132 a and the source anddrain of the transistor 133 a. The current flowing through the sourceand drain of the transistor 132 a and the source and drain of thetransistor 133 a depends on the level of the gate voltage of thetransistor 132 a. Therefore, optical data has a value corresponding tothe illuminance of light entering the photoelectric conversion element131 a. Further, the light detection circuit in FIG. 3A outputs anoptical data signal from the rest of the other of the source and drainof the transistor 132 a or the other of the source and drain of thetransistor 133 a. The above is the example of the method for driving thelight detection circuit in FIG. 3A.

Next, an example of a method for driving the light detection circuit inFIG. 3B will be described with reference to FIG. 3D. FIG. 3D is adiagram for explaining the example of the method for driving the lightdetection circuit in FIG. 3B.

In the example of the method for driving the light detection circuit inFIG. 3B, first, a pulse of the light-detection reset signal is input ina period T41. In addition, a pulse of the light-detection control signalis input in the period T41 and a period T42. Note that in the periodT41, the timing of starting input of the pulse of the light-detectionreset signal may be earlier than the timing of starting input of thepulse of the light-detection control signal.

At that time, in the period T41, the photoelectric conversion element131 b is set in the state ST51 and the transistor 134 b is turned on, sothat the gate voltage of the transistor 132 b is reset to a valueequivalent to the voltage Va.

Then, in the period T42 after the input of the pulse of thelight-detection reset signal, the photoelectric conversion element 131 bis set in the state ST52, the transistor 134 b remains on, and thetransistor 135 is turned off.

At this time, a photocurrent flows between the first current terminaland the second current terminal of the photoelectric conversion element131 b in accordance with the illuminance of light entering thephotoelectric conversion element 131 b. Further, the level of the gatevoltage of the transistor 132 b varies in accordance with thephotocurrent. At this time, the value of the channel resistance betweenthe source and the drain of the transistor 132 b is changed.

Then, in a period T43 after input of the pulse of the light-detectioncontrol signal, the transistor 134 b is turned off.

At that time, the gate voltage of the transistor 132 b is kept at avalue corresponding to the photocurrent of the photoelectric conversionelement 131 b in the period T42. Note that the period T43 is notnecessarily provided; however, in the case where there is the periodT43, the timing of outputting an optical data signal in the lightdetection circuit can be set as appropriate. For example, the timing ofoutputting an optical data signal can be set as appropriate in aplurality of light detection circuits.

Then, in a period T44, a pulse of the output selection signal is input.

At this time, the photoelectric conversion element 131 b remains in thestate ST52 and the transistor 133 b is turned on.

When the transistor 133 b is turned on, the light detection circuit inFIG. 3B outputs an optical data signal from the rest of the other of thesource and drain of the transistor 132 b or the other of the source anddrain of the transistor 133 b. A current flowing through the source anddrain of the transistor 132 b and the source and drain of the transistor133 b depends on the level of the gate voltage of the transistor 132 b.Therefore, optical data has a value corresponding to the illuminance oflight entering the photoelectric conversion element 131 b. The above isthe example of the method for driving the light detection circuit inFIG. 3B.

As described with FIGS. 3A to 3D, the light detection circuitexemplified in this embodiment includes a photoelectric conversionelement, a light-detection control transistor, and an amplificationtransistor. The light detection circuit generates optical data inaccordance with a light-detection control signal and outputs the opticaldata as a data signal in accordance with an output selection signal.With the above structure, optical data can be generated and output bythe light detection circuit.

Embodiment 4

In this embodiment, an example of a display circuit in the input-outputdevice described in Embodiments 1 and 2 will be described.

Examples of the display circuit in this embodiment will be describedwith reference to FIGS. 4A to 4D. FIGS. 4A to 4D are diagrams forexplaining an example of the display circuit in this embodiment.

First, configuration examples of the display circuit in this embodimentwill be described with reference to FIGS. 4A and 4B. FIGS. 4A and 4Beach illustrate the configuration example of the display circuit in thisembodiment.

The display circuit illustrated in FIG. 4A includes a transistor 151 a,a liquid crystal element 152 a, and a capacitor 153 a.

In the display circuit in FIG. 4A, the transistor 151 a is afield-effect transistor.

In the input-output device, a liquid crystal element includes a firstdisplay electrode, a second display electrode, and a liquid crystallayer. The light transmittance of the liquid crystal layer is changed inaccordance with a voltage applied between the first display electrodeand the second display electrode.

Further, in the input-output device, a capacitor includes a firstcapacitor electrode, a second capacitor electrode, and a dielectriclayer overlapping with the first capacitor electrode and the secondcapacitor electrode. The capacitor accumulates electric charge inaccordance with a voltage applied between the first capacitor electrodeand the second capacitor electrode.

A display data signal is input to one of a source and a drain of thetransistor 151 a. A display selection signal is input to a gate of thetransistor 151 a.

A first display electrode of the liquid crystal element 152 a iselectrically connected to the other of the source and the drain of thetransistor 151 a. A voltage Vc is input to a second display electrode ofthe liquid crystal element 152 a. The level of the voltage Vc can be setas appropriate.

A first capacitor electrode of the capacitor 153 a is electricallyconnected to the other of the source and the drain of the transistor 151a. The voltage Vc is input to a second capacitor electrode of thecapacitor 153 a.

The display circuit illustrated in FIG. 4B includes a transistor 151 b,a liquid crystal element 152 b, a capacitor 153 b, a capacitor 154, atransistor 155, and a transistor 156.

In the display circuit in FIG. 4B, the transistors 151 b, 155, and 156are field-effect transistors.

A display data signal is input to one of a source and a drain of thetransistor 155. A write selection signal (a signal WSEL) which is apulse signal is input to a gate of the transistor 155. The writeselection signal can be generated, for example, by output of a pulsesignal from a shift register included in a circuit.

A first capacitor electrode of the capacitor 154 is electricallyconnected to the other of the source and the drain of the transistor155. The voltage Vc is input to a second capacitor electrode of thecapacitor 154.

One of a source and a drain of the transistor 151 b is electricallyconnected to the other of the source and the drain of the transistor155. A display selection signal is input to a gate of the transistor 151b.

A first display electrode of the liquid crystal element 152 b iselectrically connected to the other of the source and the drain of thetransistor 151 b. The voltage Vc is input to a second display electrodeof the liquid crystal element 152 b.

A first capacitor electrode of the capacitor 153 b is electricallyconnected to the other of the source and the drain of the transistor 151b. The voltage Vc is input to a second capacitor electrode of thecapacitor 153 b. The level of the voltage Vc is set as appropriate inaccordance with specifications of the display circuit.

A reference voltage is input to one of a source and a drain of thetransistor 156. The other of the source and the drain of the transistor156 is electrically connected to the other of the source and the drainof the transistor 151 b. A display reset signal (a signal DRST) which isa pulse signal is input to a gate of the transistor 156.

Next, the components of the display circuits illustrated in FIGS. 4A and4B will be described.

The transistors 151 a and 151 b each serve as a display selectiontransistor.

As a liquid crystal layer in the liquid crystal elements 152 a and 152b, a liquid crystal layer that transmits light when a voltage applied tothe first display electrode and the second display electrode is 0 V canbe used. For example, it is possible to use a liquid crystal layerincluding electrically controlled birefringence liquid crystal (ECBliquid crystal), liquid crystal to which dichroic dye is added (GHliquid crystal), polymer-dispersed liquid crystal, or discotic liquidcrystal. Alternatively, a liquid crystal layer exhibiting a blue phasemay be used. The liquid crystal layer exhibiting a blue phase contains,for example, a liquid crystal composition including a liquid crystalexhibiting a blue phase and a chiral agent. The liquid crystalexhibiting a blue phase has a short response time of 1 msec or less andis optically isotropic; therefore, alignment treatment is not necessaryand the viewing angle dependence is small. Thus, the operation speed canbe increased with the liquid crystal layer exhibiting a blue phase. Forexample, the field sequential input-output device in Embodiments 1 and 2needs to have higher operation speed than a display device using a colorfilter, and therefore, the liquid crystal exhibiting a blue phase ispreferably used in the liquid crystalelement in the field sequentialinput-output device in Embodiments 1 and 2.

The capacitors 153 a and 153 b each serve as a storage capacitor; avoltage corresponding to a display data signal is applied between thefirst capacitor electrode and the second capacitor electrode with thetransistors 151 a and 151 b, respectively. The capacitors 153 a and 153b are not necessarily provided; in the case where the capacitors 153 aand 153 b are provided, variations of voltage applied to the liquidcrystal element due to leakage current of the display selectiontransistor can be suppressed.

The capacitor 154 serves as a storage capacitor; a voltage correspondingto a display data signal is applied between the first capacitorelectrode and the second capacitor electrode with the transistor 155.

The transistor 155 serves as a write selection transistor that selectswhether a display data signal is input to the capacitor 154.

The transistor 156 serves as a display reset selection transistor thatselects whether a voltage applied to the liquid crystal element 152 b isreset.

Examples of the transistors 151 a, 151 b, 155, and 156 are a transistorincluding a semiconductor layer containing a semiconductor that belongsto Group 14 of the periodic table (e.g., silicon) and a transistorincluding an oxide semiconductor layer; a channel is formed in thesemiconductor layer or the oxide semiconductor layer.

Next, examples of methods for driving the display circuits in FIGS. 4Aand 4B will be described.

First, an example of a method for driving the display circuit in FIG. 4Awill be described with reference to FIG. 4C. FIG. 4C is a timing chartfor explaining the example of the method for driving the display circuitin FIG. 4A and shows the states of the display data signal and thedisplay selection signal.

In the example of the method for driving the display circuit in FIG. 4A,the transistor 151 a is turned on when a pulse of the display selectionsignal is input.

When the transistor 151 a is turned on, the display data signal is inputto the display circuit, so that the voltage of the first displayelectrode of the liquid crystal element 152 a and the voltage of thefirst capacitor electrode of the capacitor 153 a become equivalent tothe voltage of the display data signal.

At this time, the liquid crystal element 152 a is put in a write state(a state wt) and has a light transmittance corresponding to the displaydata signal, so that the display circuit is put in a display statecorresponding to data (each of data D11 to data DX) of the display datasignal.

After that, the transistor 151 a is turned off, and the liquid crystalelement 152 a is put in a hold state (a state hld) and keeps the voltageapplied between the first display electrode and the second displayelectrode so that the amount of variations from the initial value doesnot exceed a reference value until a pulse of the next display selectionsignal is input. Moreover, the light unit in the input-output device inEmbodiments 1 and 2 is lit when the liquid crystal element 152 a is inthe hold state.

Next, an example of a method for driving the display circuit in FIG. 4Bwill be described with reference to FIG. 4D. FIG. 4D is a timing chartfor explaining the example of the method for driving the light detectioncircuit in FIG. 4B.

In the example of the method for driving the light detection circuit inFIG. 4B, when a pulse of the display reset signal is input, thetransistor 156 is turned on, so that the voltage of the first displayelectrode of the liquid crystal element 152 b and the first capacitorelectrode of the capacitor 153 b are reset to the reference voltage.

Moreover, when a pulse of the write selection signal is input, thetransistor 155 is turned on, whereby the display data signal is input tothe display circuit, and the voltage of the first capacitor electrode ofthe capacitor 154 becomes equivalent to the voltage of the display datasignal.

After that, when a pulse of the display selection signal is input, thetransistor 151 b is turned on, whereby the voltage of the first displayelectrode of the liquid crystal element 152 b and the voltage of thefirst capacitor electrode of the capacitor 153 b become equivalent tothe voltage of the first capacitor electrode of the capacitor 154.

At this time, the liquid crystal element 152 b is put in a write stateand has a light transmittance corresponding to the display data signal,so that the display circuit is put in a display state corresponding todata (each of data D11 to data DX) of the display data signal.

After that, the transistor 151 b is turned off, and the liquid crystalelement 152 b is put in a hold state and keeps the voltage appliedbetween the first display electrode and the second display electrode sothat the amount of variations from the initial value does not exceed areference value until a pulse of the next display selection signal isinput. Moreover, the light unit in the input-output device inEmbodiments 1 and 2 is lit when the liquid crystal element 152 b is inthe hold state.

As described with FIGS. 4A and 4B, the display circuit exemplified inthis embodiment includes a display selection transistor and a liquidcrystal element. With the above structure, the display circuit can beset in a display state corresponding to a display data signal.

In addition, as described with FIG. 4B, the display circuit exemplifiedin this embodiment includes a write selection transistor and a capacitorin addition to a display selection transistor and a liquid crystalelement. With the above structure, while the liquid crystal element isset in a display state corresponding to data of a given display datasignal, data of the next display selection signal can be written intothe capacitor. Consequently, the operation speed of the display circuitcan be increased.

Embodiment 5

In this embodiment, an example of the second light unit in theinput-output device in Embodiment 1 will be described.

A structural example of a light unit in this embodiment will bedescribed with reference to FIG. 5. FIG. 5 is a schematic viewillustrating the structural example of the light unit in thisembodiment.

The light unit illustrated in FIG. 5 includes a light source 201, alight guide plate 202, and a fixing material 203. Furthermore, the lightunit in FIG. 5 overlaps with a light detection circuit in a pixelportion (PX) 205.

As the light source 201, a light-emitting diode that emits light in thevisible light region can be used as in Embodiment 1.

The fixing material 203 has a function of fixing the light source 201and the light guide plate 202. As the fixing material 203, alight-blocking material is preferably used. The use of a light-blockingmaterial for the fixing material 203 can prevent light emitted from thelight source 201 from leaking to the outside. Note that the fixingmaterial 203 is not necessarily provided.

In the light unit illustrated in FIG. 5, light from the light source 201enters the light guide plate 202. For example, when an object to be readis not in contact with the light guide plate 202, light from the lightsource 201 is led to total reflection in the light guide plate 202. Onthe other hand, when an object to be read, such as a finger 204, is incontact with the light guide plate 202, light from the light source 201is scattered at the contact portion of the finger 204 and the lightguide plate 202 and enters the light detection circuit.

The light unit in FIG. 5 may be provided with a light control circuitwith which a lighting state and a non-lighting state are switched.

As described with FIG. 5, the light unit exemplified in this embodimentincludes a light source and a light guide plate. Light from the lightsource is led to total reflection in the light guide plate. When anobject to be read is in contact with the light guide plate, lightreflected by the object enters a light detection circuit at the contactportion. The above structure can reduce adverse effects of light in theenvironment where the input-output device is positioned.

Embodiment 6

In this embodiment, a transistor that can be applied to the transistorin the input-output device described in the above embodiment will bedescribed.

As the transistor in the input-output device described in the aboveembodiment, it is possible to use a transistor including an oxidesemiconductor layer or a semiconductor layer containing a semiconductorthat belongs to Group 14 of the periodic table (e.g., silicon), in whicha channel is formed. Note that a layer in which a channel is formed isalso referred to as a channel formation layer.

The semiconductor layer may be a single crystal semiconductor layer, apolycrystalline semiconductor layer, a microcrystalline semiconductorlayer, or an amorphous semiconductor layer.

Another example of an transistor including an oxide semiconductor layer,which is applicable to the input-output device described in the aboveembodiment, is a transistor including an oxide semiconductor layer thatbecomes intrinsic (i-type) or substantially intrinsic by purification.Note that purification means the following concepts: removal of hydrogenin an oxide semiconductor layer as much as possible, and reduction ofdefects due to oxygen vacancy in the oxide semiconductor layer by supplyof oxygen to the oxide semiconductor layer.

Examples of structures of the transistor including the oxidesemiconductor layer will be described with reference to FIGS. 6A to 6D.FIGS. 6A to 6D are schematic cross-sectional diagrams each illustratingan example of the structure of a transistor in this embodiment.

The transistor illustrated in FIG. 6A has a bottom-gate structure and isalso referred to as an inverted staggered transistor.

The transistor in FIG. 6A includes a conductive layer 401 a, aninsulating layer 402 a, an oxide semiconductor layer 403 a, a conductivelayer 405 a, and a conductive layer 406 a.

The conductive layer 401 a is provided over a substrate 400 a. Theinsulating layer 402 a is provided over the conductive layer 401 a. Theoxide semiconductor layer 403 a overlaps the conductive layer 401 a withthe insulating layer 402 a placed therebetween. The conductive layer 405a and the conductive layer 406 a are provided over part of the oxidesemiconductor layer 403 a.

In FIG. 6A, part of a top surface of the oxide semiconductor layer 403 a(part of the oxide semiconductor layer 403 a over which the conductivelayers 405 a and 406 a are not positioned) in the transistor is incontact with an oxide insulating layer 407 a.

The transistor illustrated in FIG. 6B is a channel protective(channel-stop) transistor which is one of bottom-gate transistors, andis also referred to as an inverted staggered transistor.

The transistor in FIG. 6B includes a conductive layer 401 b, aninsulating layer 402 b, an oxide semiconductor layer 403 b, a conductivelayer 405 b, a conductive layer 406 b, and an oxide insulating layer 407b.

The conductive layer 401 b is provided over a substrate 400 b. Theinsulating layer 402 b is provided over the conductive layer 401 b. Theoxide semiconductor layer 403 b overlaps the conductive layer 401 b withthe insulating layer 402 b placed therebetween. The oxide insulatinglayer 407 b is provided over the oxide semiconductor layer 403 b. Theconductive layer 405 b and the conductive layer 406 b are provided overpart of the oxide semiconductor layer 403 b with the oxide insulatinglayer 407 b placed therebetween.

The transistor illustrated in FIG. 6C is one of bottom-gate transistors.

The transistor in FIG. 6C includes a conductive layer 401 c, aninsulating layer 402 c, an oxide semiconductor layer 403 c, a conductivelayer 405 c, and a conductive layer 406 c.

The conductive layer 401 c is provided over a substrate 400 c. Theinsulating layer 402 c is provided over the conductive layer 401 c. Theconductive layer 405 c and the conductive layer 406 c are formed overpart of the insulating layer 402 c. The oxide semiconductor layer 403 coverlaps the conductive layer 401 c with the insulating layer 402 cplaced therebetween.

Further, in FIG. 6C, a top surface and a side surface of the oxidesemiconductor layer 403 c in the transistor are in contact with theoxide insulating layer 407 c.

Note that in FIGS. 6A to 6C, a protective insulating layer may beprovided over the oxide insulating layer.

The transistor illustrated in FIG. 6D is one of top-gate transistors.

The transistor in FIG. 6D includes a conductive layer 401 d, aninsulating layer 402 d, an oxide semiconductor layer 403 d, a conductivelayer 405 d, and a conductive layer 406 d.

The oxide semiconductor layer 403 d is provided over a substrate 400 dwith an insulating layer 447 placed therebetween. The conductive layer405 d and the conductive layer 406 d are provided over the oxidesemiconductor layer 403 d. The insulating layer 402 d is provided overthe oxide semiconductor layer 403 d, the conductive layer 405 d, and theconductive layer 406 d. The conductive layer 401 d overlaps the oxidesemiconductor layer 403 d with the insulating layer 402 d placedtherebetween.

Next, the components illustrated in FIGS. 6A to 6D will be described.

Each of the substrates 400 a to 400 d can be, for example, alight-transmitting substrate such as a glass substrate or a plasticsubstrate.

The insulating layer 447 functions as a base layer preventing diffusionof an impurity element from the substrate 400 d.

The insulating layer 447 can be, for example, a silicon nitride layer, asilicon oxide layer, a silicon nitride oxide layer, a silicon oxynitridelayer, an aluminum oxide layer, or an aluminum oxynitride layer.Alternatively, the insulating layer 447 can be a stack of layers ofmaterials that can be used for the insulating layer 447.

Each of the conductive layers 401 a to 401 d functions as a gate of thetransistor. Note that a layer functioning as a gate of the transistorcan be called a gate electrode or a gate wiring.

Note that the transistor in this embodiment may include a conductivelayer that overlaps the conductive layer to be the gate with the oxidesemiconductor layer placed therebetween, in addition to the componentsof the transistor illustrated in FIGS. 6A to 6D. The conductive layeralso functions as a gate of the transistor. With the above structure,the threshold voltage of the transistor can be controlled, and entry oflight into the oxide semiconductor layer can be prevented.

As the conductive layers 401 a to 401 d, it is possible to use, forexample, a layer of a metal material such as molybdenum, titanium,chromium, tantalum, tungsten, aluminum, copper, neodymium, or scandiumor an alloy material containing any of these materials as a maincomponent. The conductive layers 401 a to 401 d can also be formed bystacking layers of materials that can be used for the conductive layers401 a to 401 d.

Each of the insulating layers 402 a to 402 d functions as a gateinsulating layer of the transistor. Note that a layer functioning as agate insulating layer of the transistor can be called a gate insulatinglayer.

As the insulating layers 402 a to 402 c, a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, an aluminum oxide layer, an aluminum nitride layer, analuminum oxynitride layer, an aluminum nitride oxide layer, or a hafniumoxide layer can be used, for example. The insulating layers 402 a to 402c can also be formed by stacking layers of materials that can be usedfor the insulating layers 402 a to 402 c. Moreover, the insulating layer402 d can be an oxide insulating layer such as a silicon oxide layer.

Each of the oxide semiconductor layers 403 a to 403 d serves as a layerin which a channel of the transistor is formed. Examples of an oxidesemiconductor applicable to the oxide semiconductor layers 403 a to 403d include at least one element selected from In, Ga, Sn, Zn, Al, Mg, Hf,and lanthanoid. For example, examples of the oxide semiconductorapplicable to the oxide semiconductor layers 403 a to 403 d are an oxideof four metal elements, an oxide of three metal elements, and an oxideof two metal elements. As the oxide of four metal elements, anIn—Sn—Ga—Zn—O-based metal oxide can be used, for example. As the oxideof three metal elements, an In—Ga—Zn—O-based metal oxide, anIn—Sn—Zn—O-based metal oxide, an In—Al—Zn—O-based metal oxide, aSn—Ga—Zn—O-based metal oxide, an Al—Ga—Zn—O-based metal oxide, aSn—Al—Zn—O-based metal oxide, an In—Hf—Zn—O-based metal oxide, anIn—La—Zn—O-based metal oxide, an In—Ce—Zn—O-based metal oxide, anIn—Pr—Zn—O-based metal oxide, an In—Nd—Zn—O-based metal oxide, anIn—Pm—Zn—O-based metal oxide, an In—Sm—Zn—O-based metal oxide, anIn—Eu—Zn—O-based metal oxide, an In—Gd—Zn—O-based metal oxide, anIn—Tb—Zn—O-based metal oxide, an In—Dy—Zn—O-based metal oxide, anIn—Ho—Zn—O-based metal oxide, an In—Er—Zn—O-based metal oxide, anIn—Tm—Zn—O-based metal oxide, an In—Yb—Zn—O-based metal oxide, or anIn—Lu—Zn—O-based metal oxide can be used, for example. As the oxide oftwo metal elements, an In—Zn—O-based metal oxide, a Sn—Zn—O-based metaloxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-based metal oxide, aSn—Mg—O-based metal oxide, an In—Mg—O-based metal oxide, anIn—Sn—O-based metal oxide, or an In—Ga—O-based metal oxide can be used,for example. In addition, an In—O-based metal oxide, a Sn—O-based metaloxide, a Zn—O-based metal oxide, or the like can also be used as theoxide semiconductor. Further, the metal oxide that can be used as theoxide semiconductor may contain silicon oxide.

In the case of using an In—Zn—O-based metal oxide, a semiconductor layerof an In—Zn—O-based metal oxide can be formed, for example, using anoxide target that has a composition ratio of In:Zn=50:1 to 1:2 in anatomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferablyIn:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molarratio), further preferably In:Zn=15:1 to 1.5:1 (In₂O₃:ZnO=15:2 to 3:4 ina molar ratio). For example, when the atomic ratio of the target usedfor forming the In—Zn—O-based oxide semiconductor is expressed byIn:Zn:O=P:Q:R, the relation of R>1.5P+Q is satisfied. An increase in theamount of indium can increase the mobility of the transistor.

As the oxide semiconductor, a material represented by InMO₃(ZnO)_(m) (mis larger than 0) can also be used. Here, M in InMO₃(ZnO)_(m) representsone or more metal elements selected from Ga, Al, Mn, and Co.

The conductive layers 405 a to 405 d and the conductive layers 406 a to406 d function as a source or a drain of the transistor. Note that alayer functioning as a source of the transistor can be called a sourceelectrode or a source wiring, and a layer functioning as a drain of thetransistor can be called a drain electrode or a drain wiring.

Each of the conductive layers 405 a to 405 d and the conductive layers406 a to 406 d can be, for example, a layer of a metal material such asaluminum, chromium, copper, tantalum, titanium, molybdenum, or tungstenor an alloy material containing any of these metal materials as a maincomponent. Alternatively, each of the conductive layers 405 a to 405 dand the conductive layers 406 a to 406 d can be a stack of layers ofmaterials applicable to the conductive layers 405 a to 405 d and theconductive layers 406 a to 406 d.

Alternatively, the conductive layers 405 a to 405 d and the conductivelayers 406 a to 406 d can be formed using a layer containing conductivemetal oxide. Examples of the conductive metal oxide are indium oxide,tin oxide, zinc oxide, an alloy of indium oxide and tin oxide, and analloy of indium oxide and zinc oxide. Note that the conductive metaloxide applicable to the conductive layers 405 a to 405 d and theconductive layers 406 a to 406 d may contain silicon oxide.

As the oxide insulating layers 407 a to 407 c, a silicon oxide layer canbe used, for example. Note that the oxide insulating layer 407 b servesas a layer protecting a channel formation layer of the transistor (alsoreferred to as a channel protective layer).

Note that the transistor in this embodiment does not necessarily havethe structure where the entire oxide semiconductor layer overlaps withthe conductive layer serving as a gate electrode as illustrated in FIGS.6A to 6D; in the case of employing the structure where the entire oxidesemiconductor layer overlaps with the conductive layer serving as a gateelectrode, entry of light into the oxide semiconductor layer can beprevented.

Next, as an example of a method for manufacturing the transistor in thisembodiment, an example of a method for manufacturing the transistor inFIG. 6A will be described with reference to FIGS. 7A to 7E. FIGS. 7A to7E are schematic cross-sectional views illustrating an example of amethod for manufacturing the transistor in FIG. 6A.

First, as illustrated in FIG. 7A, the substrate 400 a is prepared, afirst conductive film is formed over the substrate 400 a, and part ofthe first conductive film is etched to form the conductive layer 401 a.

For example, the first conductive film can be formed by formation of afilm of a material applicable to the conductive layer 401 a bysputtering. Alternatively, the first conductive film can be formed bystacking films of materials that can be used for the conductive layer401 a.

When a high-purity gas from which impurities such as hydrogen, water, ahydroxyl group, or a hydride are removed is used as a sputtering gas,the impurity concentration of a film to be formed can be reduced.

Note that before the film is formed by sputtering, preheat treatment maybe performed in a preheating chamber of a sputtering apparatus. By thepreheat treatment, impurities such as hydrogen or moisture can beeliminated.

Moreover, before the film is formed by sputtering, it is possible toperform the following treatment (called reverse sputtering): instead ofapplying a voltage to the target side, an RF power source is used forapplying a voltage to the substrate side in an argon, nitrogen, helium,or oxygen atmosphere so that plasma is generated to modify a surfacewhere the film is to be formed. With reverse sputtering, powderysubstances (also referred to as particles or dust) attached to thesurface where the film is to be formed can be removed.

In the case where the film is formed by sputtering, moisture remainingin a deposition chamber used for forming the film can be removed with anentrapment vacuum pump. As the entrapment vacuum pump, a cryopump, anion pump, or a titanium sublimation pump can be used, for example.Moreover, moisture remaining in the deposition chamber can be removedwith a turbo pump provided with a cold trap.

Alternatively, the conductive layer 401 a can be formed in the followingmanner, for example: a resist mask is formed over part of the firstconductive film in a photolithography process and the first conductivefilm is etched using the resist mask. In that case, the resist mask isremoved after the conductive layer 401 a is formed.

Note that the resist mask may be formed by an inkjet method. Since aninkjet method does not need a photomask, manufacturing costs can bereduced. Alternatively, the resist mask may be formed using alight-exposure mask having a plurality of regions with differenttransmittances (also referred to as a multi-tone mask). With amulti-tone mask, a resist mask having different thicknesses can beformed, and the number of resist masks used for manufacturing thetransistor can be reduced.

Next, as illustrated in FIG. 7B, the insulating layer 402 a is formed byformation of a first insulating film over the conductive layer 401 a.

For example, the first insulating film can be formed by formation of afilm of a material applicable to the insulating layer 402 a bysputtering, plasma CVD, or the like. The first insulating film can alsobe formed by stacking films of materials that can be used for theinsulating layer 402 a. Moreover, when a film of a material applicableto the insulating layer 402 a is formed by high-density plasma CVD(e.g., high-density plasma CVD using microwaves at a frequency of 2.45GHz), the insulating layer 402 a can be dense and has an improvedbreakdown voltage.

Next, an oxide semiconductor film is formed over the insulating layer402 a and then part of the oxide semiconductor film is etched, wherebythe oxide semiconductor layer 403 a is formed as illustrated in FIG. 7C.

For example, the oxide semiconductor film can be formed by formation ofa film of an oxide semiconductor material applicable to the oxidesemiconductor layer 403 a by sputtering. Note that the oxidesemiconductor film may be formed in a rare gas atmosphere, an oxygenatmosphere, or a mixed atmosphere of a rare gas and oxygen.

The oxide semiconductor film can be formed using an oxide target havinga composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] as asputtering target. Alternatively, the oxide semiconductor film may beformed using an oxide target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio], for example.

When the oxide semiconductor film is formed by sputtering, the substrate400 a may be placed under reduced pressure and heated to 100 C.° to 600C.°, preferably 200 C.° to 400 C.°. By heating the substrate 400 a, theconcentration of impurities in the oxide semiconductor film can bereduced and damage to the oxide semiconductor film during the sputteringcan be reduced.

Alternatively, the oxide semiconductor layer 403 a can be formed in thefollowing manner, for example: a resist mask is formed over part of theoxide semiconductor film in a photolithography process and the oxidesemiconductor film is etched using the resist mask. In that case, theresist mask is removed after the oxide semiconductor film is etched.

Next, as illustrated in FIG. 7D, a second conductive film is formed overthe insulating layer 402 a and the oxide semiconductor layer 403 a, andpart of the second conductive film is etched to form the conductivelayers 405 a and 406 a.

For example, the second conductive film can be formed by formation of afilm of a material applicable to the conductive layers 405 a and 406 aby sputtering. Alternatively, the second conductive film can be formedby stacking films of materials applicable to the conductive layers 405 aand 406 a.

Alternatively, the conductive layers 405 a and 406 a can be formed inthe following manner, for example: a resist mask is formed over part ofthe second conductive film in a photolithography process and the secondconductive film is etched using the resist mask. In that case, theresist mask is removed after the conductive layers 405 a and 406 a areformed.

Then, as illustrated in FIG. 7E, the oxide insulating layer 407 a isformed so as to be contact with the oxide semiconductor layer 403 a.

For example, the oxide insulating layer 407 a can be formed by formationof a film applicable to the oxide insulating layer 407 a by sputteringin a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or amixed atmosphere of a rare gas and oxygen. The oxide insulating layer407 a formed by sputtering can suppress a reduction in resistance of aportion of the oxide semiconductor layer 403 a, which serves as a backchannel of the transistor. The temperature of the substrate at the timewhen the oxide insulating layer 407 a is formed preferably ranges fromroom temperature to 300 C.°.

Before formation of the oxide insulating layer 407 a, plasma treatmentwith the use of a gas such as N₂O, N₂, or Ar may be performed to removewater or the like adsorbed on an exposed surface of the oxidesemiconductor layer 403 a. In the case of performing the plasmatreatment, the oxide insulating layer 407 a is preferably formed afterthe plasma treatment without exposure to air.

Further, in the example of the method for manufacturing the transistorin FIG. 6A, heat treatment is performed, for example, at 400° C. orhigher and 750° C. or lower, or 400° C. or higher and lower than thestrain point of the substrate. For example, the heat treatment isperformed after the oxide semiconductor film is formed, after part ofthe oxide semiconductor film is etched, after the second conductive filmis formed, after part of the second conductive film is etched, or afterthe oxide insulating layer 407 a is formed.

A heat treatment apparatus for the heat treatment can be an electricfurnace or an apparatus for heating an object by heat conduction or heatradiation from a heating element such as a resistance heating element.For example, a rapid thermal annealing (RTA) apparatus such as a gasrapid thermal annealing (GRTA) apparatus or a lamp rapid thermalannealing (LRTA) apparatus can be used. An LRTA apparatus is anapparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As thehigh-temperature gas, a rare gas or an inert gas (e.g., nitrogen) thatdoes not react with an object by the heat treatment can be used.

Further, after the heat treatment, a high-purity oxygen gas, ahigh-purity N₂O gas, or ultra-dry air (having a dew point −40° C. orlower, preferably −60° C. or lower) may be introduced in the furnacewhere the heat treatment has been performed while the heatingtemperature is maintained or the temperature is decreased from theheating temperature. In that case, it is preferable that the oxygen gasor the N₂O gas do not contain water, hydrogen, and the like. The purityof the oxygen gas or the N₂O gas which is introduced into the heattreatment apparatus is preferably 6N or more, further preferably 7N ormore (i.e., the impurity concentration of the oxygen gas or the N₂O gasis preferably 1 ppm or lower, further preferably 0.1 ppm or lower). Bythe effect of the oxygen gas or the N₂O gas, oxygen is supplied to theoxide semiconductor layer 403 a, so that the oxide semiconductor layer403 a can be purified.

Besides the above heat treatment, heat treatment (preferably at 200° C.to 400° C., for example at 250° C. to 350° C.) may be performed in aninert gas atmosphere or an oxygen gas atmosphere after the oxideinsulating layer 407 a is formed.

Oxygen doping using oxygen plasma may be performed after the insulatinglayer 402 a is formed, after the oxide semiconductor film is formed,after the conductive layers serving as the source electrode and thedrain electrode are formed, after the oxide insulating layer is formed,or after the heat treatment is performed. For example, oxygen doping maybe performed using a high-density plasma of 2.45 GHz. The oxygen dopingcan reduce variations in electrical characteristics of transistors to bemanufactured.

Through the above steps, impurities such as hydrogen, moisture, ahydroxyl group, or a hydride (also referred to as a hydrogen compound)are removed from the oxide semiconductor layer 403 a, and in addition,oxygen is supplied to the oxide semiconductor layer 403 a by the effectof the oxygen gas or the N₂O gas.

Consequently, defects due to oxygen vacancy in the oxide semiconductorlayer 403 a can be reduced.

The example of the method for manufacturing the transistor, which isshown in this embodiment, does not necessarily apply only to thetransistor in FIG. 6A. For example, if any of the components illustratedin FIGS. 6B to 6D has the same designation as the components in FIG. 6Aand has a function, at least part of which is the same as that of thecomponents in FIG. 6A, the description of the example of the method formanufacturing the transistor in FIG. 6A can be employed as appropriate.

As described with FIGS. 6A to 6D and FIGS. 7A to 7E, the transistorexemplified in this embodiment includes a conductive layer serving as agate; an insulating layer serving as a gate insulating layer; an oxidesemiconductor layer that overlaps the conductive layer serving as thegate, with the insulating layer serving as the gate insulating layerplaced therebetween, in which a channel is formed; a conductive layerthat is electrically connected to the oxide semiconductor layer andserves as one of a source and a drain; and a conductive layer that iselectrically connected to the oxide semiconductor layer and serves asthe other of the source and the drain.

The oxide semiconductor layer in which the channel is formed is an oxidesemiconductor layer that is made to be intrinsic (i-type) orsubstantially intrinsic by purification. By purification of the oxidesemiconductor layer, the carrier concentration of the oxidesemiconductor layer can be lower than 1×10¹⁴/cm³, preferably lower than1×10¹²/cm³, further preferably lower than 1×10¹¹/cm³; thus, variation incharacteristics due to temperature change can be suppressed. With theabove structure, the off-state current of the transistor per 1 μm ofchannel width can be reduced to 10 aA (1×10⁻¹⁷ A) or lower, 1 aA(1×10⁻¹⁸ A) or lower, 10 zA (1×10⁻²⁰ A) or lower, further reduced to 1zA (1×10⁻²¹ A) or lower, and still further reduced to 100 yA (1×10⁻²² A)or lower. It is preferable that the off-state current of the transistorbe as low as possible. The lowest value of the off-state current of thetransistor in this embodiment is estimated to be about 10⁻³⁰ A/μm.

Next, as for the transistor including the oxide semiconductor layerexemplified in this embodiment, an example of calculating the off-statecurrent by leakage current measurement with a circuit for evaluatingcharacteristics will be described below.

Leakage current measurement using a circuit for evaluatingcharacteristics will be described with reference to FIGS. 8A and 8B.FIGS. 8A and 8B are diagrams for explaining a circuit for evaluatingcharacteristics.

First, a configuration of a circuit for evaluating characteristics isdescribed with reference to FIG. 8A. FIG. 8A is a circuit diagramillustrating the configuration of the circuit for evaluatingcharacteristics.

The circuit for evaluating characteristics illustrated in FIG. 8Aincludes a plurality of measurement systems 801. The plurality ofmeasurement systems 801 are connected in parallel with each other. Here,as an example, eight measurement systems 801 are connected in parallelwith each other. By using the plurality of measurement systems 801, aplurality of leakage currents can be measured at the same time.

The measurement system 801 includes a transistor 811, a transistor 812,a capacitor 813, a transistor 814, and a transistor 815.

A voltage V1 is input to one of a source and a drain of the transistor811. A voltage Vext_a is input to a gate of the transistor 811. Thetransistor 811 is a transistor for injecting charge.

One of a source and a drain of the transistor 812 is connected to theother of the source and the drain of the transistor 811. A voltage V2 isinput to the other of the source and the drain of the transistor 812. Avoltage Vext_b is input to a gate of the transistor 812. The transistor812 is a transistor for evaluating leakage current. Note that “leakagecurrent” in this embodiment refers to a leakage current including anoff-state current of the transistor.

A first capacitor electrode of the capacitor 813 is connected to theother of the source and the drain of the transistor 811. The voltage V2is input to a second capacitor electrode of the capacitor 813. Here, thevoltage V2 is 0 V.

A voltage V3 is input to one of a source and a drain of the transistor814. A gate of the transistor 814 is connected to the other of thesource and the drain of the transistor 811. Note that a portion wherethe gate of the transistor 814, the other of the source and the drain ofthe transistor 811, the one of the source and the drain of thetransistor 812, and the first capacitor electrode of the capacitor 813are connected to each other is referred to as a node A. Here, thevoltage V3 is 5 V.

One of a source and a drain of the transistor 815 is connected to theother of the source and the drain of the transistor 814. A voltage V4 isinput to the other of the source and the drain of the transistor 815. Avoltage Vext_c is input to a gate of the transistor 815. Here, thevoltage Vext_c is 0.5 V.

The measurement system 801 outputs a voltage at a portion where theother of the source and the drain of the transistor 814 is connected tothe one of the source and the drain of the transistor 815, as an outputvoltage Vout.

Here, as an example of the transistor 811, a transistor that includes anoxide semiconductor layer and has a channel length L of 10 μm and achannel width W of 10 μm is used. As an example of the transistors 814and 815, a transistor that includes an oxide semiconductor layer and hasa channel length L of 3 μm and a channel width W of 100 μm is used.Moreover, as an example of the transistor 812, a bottom-gate transistorthat includes an oxide semiconductor layer is used. In the transistor, asource electrode and a drain electrode are in contact with an upper partof the oxide semiconductor layer, a region where the source and drainelectrodes overlap with a gate electrode is not provided, and an offsetregion with a width of 1 μm is provided. By providing the offset region,parasitic capacitance can be reduced. Further, as the transistor 812,six samples (SMP) of transistors having different channel lengths L andchannel widths W are used (see Table 1).

TABLE 1 L [μm] W [μm] SMP1 1.5 1 × 10⁵ SMP2 3 1 × 10⁵ SMP3 10 1 × 10⁵SMP4 1.5 1 × 10⁶ SMP5 3 1 × 10⁶ SMP6 10 1 × 10⁶

The transistor for injecting charge and the transistor for evaluatingleakage current are separately provided as illustrated in FIG. 8A, sothat the transistor for evaluating leakage current can be always keptoff while electric charge is injected. In the case where the transistorfor injecting charge is not provided, the transistor for evaluatingleakage current needs to be turned on once at the time of chargeinjection. In that case, if the transistor requires a long time to reacha steady off state from an on state, it takes a long time formeasurement.

In addition, the transistor for injecting charge and the transistor forevaluating leakage current are separately provided, whereby eachtransistor can have an appropriate size. When the channel width W of thetransistor for evaluating leakage current is made larger than that ofthe transistor for injecting charge, leakage current components of thecircuit for evaluating characteristics other than the leakage current ofthe transistor for evaluating leakage current can be made relativelysmall. As a result, the leakage current of the transistor for evaluatingleakage current can be measured with high accuracy. In addition, sincethe transistor for evaluating leakage current does not need to be turnedon at the time of charge injection, the measurement is not adverselyaffected by variation in the voltage of the node A, which is caused whenpart of electric charge in the channel formation region flows into thenode A.

On the other hand, when the channel width W of the transistor forinjecting charge is made smaller than that of the transistor forevaluating leakage current, the leakage current of the transistor forinjecting charge can be relatively small. Further, the measurement isless adversely affected by variation in the voltage of the node A, whichis caused when part of charge in the channel formation region flows intothe node A at the time of charge injection.

Next, a method for measuring a leakage current of the circuit forevaluating characteristics illustrated in FIG. 8A will be described withreference to FIG. 8B. FIG. 8B is a timing chart for explaining themethod for measuring a leakage current with use of the circuit forevaluating characteristics illustrated in FIG. 8A.

In the method for measuring the leakage current with the circuit forevaluating characteristics illustrated in FIG. 8A, a write period and ahold period are provided. The operation in each period is describedbelow.

In the write period, a voltage VL (−3 V) with which the transistor 812is turned off is input as the voltage Vext_b. Moreover, a write voltageVw is input as the voltage V1, and then, a voltage VH (5 V) with whichthe transistor 811 is turned on is input as the voltage Vext_a for agiven period. Thus, electric charge is accumulated in the node A, andthe voltage of the node A becomes equivalent to the write voltage Vw.Then, the voltage VL with which the transistor 811 is turned off isinput as the voltage Vext_a. After that, a voltage VSS (0 V) is input asthe voltage V1.

In the hold period, the amount of change in the voltage of the node A,caused by change in the amount of the electric charge held in the nodeA, is measured. From the amount of change in the voltage, the value ofthe current flowing between the source electrode and the drain electrodeof the transistor 812 can be calculated. In the above manner, electriccharge can be accumulated in the node A, and the amount of change in thevoltage of the node A can be measured.

Accumulation of electric charge in the node A and measurement of theamount of change in the voltage of the node A (also referred to as anaccumulation and measurement operation) are repeatedly performed. First,a first accumulation and measurement operation is repeated 15 times. Inthe first accumulation and measurement operation, a voltage of 5 V isinput as the write voltage Vw in the write period and retained for 1hour in the hold period. Next, a second accumulation and measurementoperation is repeated twice. In the second accumulation and measurementoperation, a voltage of 3.5 V is input as the write voltage Vw in thewrite period and retained for 50 hours in the hold period. Next, a thirdaccumulation and measurement operation is performed once. In the thirdaccumulation and measurement operation, a voltage of 4.5 V is input asthe write voltage Vw in the write period and retained for 10 hours inthe hold period. By repeating the accumulation and measurementoperation, the measured current value can be confirmed to be the valuein the steady state. In other words, the transient current (a currentcomponent that decreases over time after the measurement starts) can beremoved from a current I_(A) flowing through the node A. Consequently,the leakage current can be measured with higher accuracy.

In general, a voltage V_(A) of the node A is expressed as a function ofthe output voltage Vout by Equation 1.

V _(A) =F(Vout)  [Equation 1]

Electric charge Q_(A) of the node A is expressed by Equation 2, usingthe voltage V_(A) of the node A, capacitance C_(A) connected to the nodeA, and a constant (const). Here, the capacitance C_(A) connected to thenode A is the sum of the capacitance of the capacitor 813 and acapacitance other than that of the capacitor 813.

Q _(A) =C _(A) V _(A)+const  [Equation 2]

Since the current I_(A) of the node A is the time differential ofelectric charge flowing into the node A (or electric charge flowing fromthe node A), the current I_(A) of the node A is expressed by Equation 3.

$\begin{matrix}{{I_{A} \equiv \frac{\Delta \; Q_{A}}{\Delta \; t}} = \frac{C_{A}^{*}\Delta \; {F({Vout})}}{\Delta \; t}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Here, as an example, Δt is about 54000 sec. As above, the current I_(A)of the node A, which is the leakage current, can be calculated with thecapacitance C_(A) connected to the node A and the output voltage Vout,so that the leakage current of the circuit for evaluatingcharacteristics can be obtained.

Next, the results of measuring the output voltage by the measurementmethod using the above circuit for evaluating characteristics and thevalue of the leakage current of the circuit for evaluatingcharacteristics, which is calculated from the measurement results, willbe described with reference to FIGS. 9A and 9B.

As an example, FIG. 9A shows the relation between the elapsed time Timeof the above measurement (the first accumulation and measurementoperation) of the transistors SMP4, SMP5, and SMP6 and the outputvoltage Vout. FIG. 9B shows the relation between the elapsed time Timeof the above measurement and the current I_(A) calculated by themeasurement. It is found that the output voltage Vout varies after themeasurement starts and it takes 10 hours or longer to reach a steadystate.

FIG. 10 shows the relation between the voltage of the node A in SMP1 toSMP6 and the leakage current estimated by the above measurement. In SMP4in FIG. 10, for example, when the voltage of the node A is 3.0 V, theleakage current is 28 yA/μm. Since the leakage current includes theoff-state current of the transistor 812, the off-state current of thetransistor 812 can be considered to be 28 yA/μm or less.

FIG. 11, FIG. 12, and FIG. 13 each show the relation between the voltageof the node A in SMP1 to SMP6 at 85° C., 125° C., and 150° C. and theleakage current estimated by the above measurement. As shown in FIGS. 11to 13, the leakage current is 100 zA/μm or less even at 150° C.

As described above, the leakage current of the circuit for evaluatingcharacteristics, including the transistor including a purified oxidesemiconductor layer serving as a channel formation layer is sufficientlylow, which means that the off-state current of the transistor issufficiently low. In addition, the off-state current of the transistoris sufficiently low even when the temperature rises.

Embodiment 7

In this embodiment, a structural example of the input-output device inthe above embodiment will be described.

An input-output device in this embodiment includes a first substrate (anactive matrix substrate) where a semiconductor element such as atransistor is provided, a second substrate, and a liquid crystal layerprovided between the first substrate and the second substrate.

First, a structural example of the active matrix substrate in thisembodiment will be described with reference to FIGS. 14A and 14B andFIGS. 15A and 15B. FIGS. 14A and 14B and FIGS. 15A and 15B illustrate astructural example of the active matrix substrate in the input-outputdevice of this embodiment. FIG. 14A is a schematic plan view. FIG. 14Bis a schematic cross-sectional view along line A-B in FIG. 14A. FIG. 15Ais a schematic plan view. FIG. 15B is a schematic cross-sectional viewalong line C-D in FIG. 15A. Note that FIGS. 15A and 15B illustrate alight detection circuit having the structure in FIG. 3A, as an example.In addition, FIGS. 14A and 14B and FIGS. 15A and 15B illustrate atransistor having the structure in FIG. 6A, as an example.

The active matrix substrate illustrated in FIGS. 14A and 14B and FIGS.15A and 15B includes a substrate 500, conductive layers 501 a to 501 h,an insulating layer 502, semiconductor layers 503 a to 503 d, conductivelayers 504 a to 504 k, an insulating layer 505, a semiconductor layer506, a semiconductor layer 507, a semiconductor layer 508, an insulatinglayer 509, and conductive layers 510 a to 510 c.

Each of the conductive layers 501 a to 501 h is formed over one surfaceof the substrate 500.

The conductive layer 501 a functions as a gate of a display selectiontransistor in a display circuit.

The conductive layer 501 b functions as a first capacitor electrode of astorage capacitor in the display circuit. Note that the layer serving asa first capacitor electrode of a capacitor (a storage capacitor) is alsoreferred to as a first capacitor electrode.

The conductive layer 501 c functions as a wiring to which the voltage Vbis input. Note that a layer having a function of a wiring can bereferred to as a wiring.

The conductive layer 501 d functions as a gate of a light-detectioncontrol transistor in the light detection circuit.

The conductive layer 501 e functions as a signal line to which thelight-detection control signal is input. Note that a layer having afunction of a signal line can be referred to as a signal line.

The conductive layer 501 f functions as a gate of an output selectiontransistor in the light detection circuit.

The conductive layer 501 g functions as a gate of an amplificationtransistor in the light detection circuit.

The insulating layer 502 is provided over the one surface of thesubstrate 500 with the conductive layers 501 a to 501 h placedtherebetween.

The insulating layer 502 functions as a gate insulating layer of thedisplay selection transistor in the display circuit, a dielectric layerof the storage capacitor in the display circuit, a gate insulating layerof the light-detection control transistor in the light detectioncircuit, a gate insulating layer of the amplification transistor in thelight detection circuit, and a gate insulating layer of the outputselection transistor in the light detection circuit.

The semiconductor layer 503 a overlaps the conductive layer 501 a withthe insulating layer 502 placed therebetween. The semiconductor layer503 a functions as a channel formation layer of the display selectiontransistor in the display circuit.

The semiconductor layer 503 b overlaps the conductive layer 501 d withthe insulating layer 502 placed therebetween. The semiconductor layer503 b functions as a channel formation layer of the light-detectioncontrol transistor in the light detection circuit.

The semiconductor layer 503 c overlaps the conductive layer 501 f withthe insulating layer 502 placed therebetween. The semiconductor layer503 c functions as a channel formation layer of the output selectiontransistor in the light detection circuit.

The semiconductor layer 503 d overlaps the conductive layer 501 g withthe insulating layer 502 placed therebetween. The semiconductor layer503 d functions as a channel formation layer of the amplificationtransistor in the light detection circuit.

The conductive layer 504 a is electrically connected to thesemiconductor layer 503 a. The conductive layer 504 a functions as oneof a source and a drain of the display selection transistor in thedisplay circuit.

The conductive layer 504 b is electrically connected to the conductivelayer 501 b and the semiconductor layer 503 a. The conductive layer 504b functions as the other of the source and the drain of the displayselection transistor in the display circuit.

The conductive layer 504 c overlaps the conductive layer 501 b with theinsulating layer 502 placed therebetween. The conductive layer 504 cfunctions as a second capacitor electrode of the storage capacitor inthe display circuit.

The conductive layer 504 d is electrically connected to the conductivelayer 501 c in an opening portion that penetrates the insulating layer502. The conductive layer 504 d functions as the one of a first currentterminal and a second current terminal of a photoelectric conversionelement in the light detection circuit.

The conductive layer 504 e is electrically connected to thesemiconductor layer 503 b. The conductive layer 504 e functions as oneof a source and a drain of the light-detection control transistor in thelight detection circuit.

The conductive layer 504 f is electrically connected to thesemiconductor layer 503 b and is electrically connected to theconductive layer 501 g in an opening portion that penetrates theinsulating layer 502. The conductive layer 504 f functions as the otherof the source and the drain of the light-detection control transistor inthe light detection circuit.

The conductive layer 504 g is electrically connected to the conductivelayers 501 d and 501 e in opening portions that penetrate the insulatinglayer 502. The conductive layer 504 g functions as a signal line towhich the light-detection control signal is input.

The conductive layer 504 h is electrically connected to thesemiconductor layer 503 c. The conductive layer 504 h functions as oneof a source and a drain of the output selection transistor in the lightdetection circuit.

The conductive layer 504 i is electrically connected to thesemiconductor layers 503 c and 503 d. The conductive layer 504 ifunctions as the other of the source and the drain of the outputselection transistor in the light detection circuit and one of a sourceand a drain of the amplification transistor in the light detectioncircuit.

The conductive layer 504 j is electrically connected to thesemiconductor layer 503 d and is electrically connected to theconductive layer 501 h in an opening portion that penetrates theinsulating layer 502. The conductive layer 504 j functions as the otherof the source and the drain of the amplification transistor in the lightdetection circuit.

The conductive layer 504 k is electrically connected to the conductivelayer 501 h in an opening portion that penetrates the insulating layer502. The conductive layer 504 k functions as a wiring to which thevoltage Va or the voltage Vb is input.

The insulating layer 505 is in contact with the semiconductor layers 503a to 503 d with the conductive layers 504 a to 504 k placedtherebetween.

The semiconductor layer 506 is electrically connected to the conductivelayer 504 d in an opening portion that penetrates the insulating layer505.

The semiconductor layer 507 is in contact with the semiconductor layer506.

The semiconductor layer 508 is in contact with the semiconductor layer507.

The insulating layer 509 overlaps the insulating layer 505, thesemiconductor layer 506, the semiconductor layer 507, and thesemiconductor layer 508. The insulating layer 509 functions as aplanarization insulating layer in the display circuit and the lightdetection circuit. Note that the insulating layer 509 is not necessarilyprovided.

The conductive layer 510 a is electrically connected to the conductivelayer 504 b in an opening portion that penetrates the insulating layers505 and 509. The conductive layer 510 a functions as a pixel electrodeof a display element in the display circuit. Note that a layer having afunction of a pixel electrode can be referred to as a pixel electrode.

The conductive layer 510 b is electrically connected to the conductivelayer 504 c in an opening portion that penetrates the insulating layers505 and 509. The conductive layer 510 b functions as a wiring to whichthe voltage Vc is input.

The conductive layer 510 c is electrically connected to the conductivelayer 504 e in an opening portion that penetrates the insulating layers505 and 509, and is electrically connected to the semiconductor layer508 in an opening portion that penetrates the insulating layers 505 and509.

Next, a structural example of the input-output device in this embodimentwill be described with reference to FIGS. 16A and 16B. FIGS. 16A and 16Bare schematic cross-sectional views illustrating a structural example ofthe input-output device in this embodiment. FIG. 16A is a schematiccross-sectional view of a display circuit. FIG. 16B is a schematiccross-sectional view of a light detection circuit. Note that a displayelement in FIGS. 16A and 16B is a liquid crystal element as an example.

The input-output device illustrated in FIGS. 16A and 16B includes asubstrate 512, a conductive layer 513, and a liquid crystal layer 514 inaddition to the active matrix substrate illustrated in FIGS. 14A and 14Band FIGS. 15A and 15B.

The conductive layer 513 is provided on one surface of the substrate512. The conductive layer 513 functions as a common electrode of thedisplay circuit. Note that the conductive layer 513 is not necessarilyprovided in the light detection circuit.

The liquid crystal layer 514 is provided between the conductive layer510 a and the conductive layer 513 and overlaps the semiconductor layer508 with the insulating layer 509 placed therebetween.

The conductive layer 510 a, the liquid crystal layer 514, and theconductive layer 513 function as a display element in the displaycircuit.

Next, the components of the input-output device illustrated in FIGS. 16Aand 16B will be described.

As the substrate 500 and the substrate 512, a substrate that can beapplied to the substrate 400 a in FIG. 6A can be used.

As the conductive layers 501 a to 501 h, a layer of a materialapplicable to the conductive layer 401 a in FIG. 6A can be used.Alternatively, the conductive layers 501 a to 501 h may be formed bystacking layers of materials applicable to the conductive layer 401 a.

As the insulating layer 502, a layer of a material applicable to theinsulating layer 402 a in FIG. 6A can be used. Alternatively, theinsulating layer 502 may be formed by stacking layers of materialsapplicable to the insulating layer 402 a.

As the semiconductor layers 503 a to 503 d, a layer of a materialapplicable to the oxide semiconductor layer 403 a in FIG. 6A can beused. Alternatively, a semiconductor layer using a semiconductorbelonging to Group 14 of the periodic table (e.g., silicon) may be usedas the semiconductor layers 503 a to 503 d.

As the conductive layers 504 a to 504 k, a layer of a materialapplicable to the conductive layer 405 a or the conductive layer 406 aillustrated in FIG. 6A can be used. Alternatively, the conductive layers504 a to 504 k may be formed by stacking layers of materials applicableto the conductive layer 405 a or the conductive layer 406 a.

As the insulating layer 505, a layer of a material applicable to theoxide insulating layer 407 a in FIG. 6A can be used. Alternatively, theinsulating layer 505 may be formed by stacking layers of materialsapplicable to the oxide insulating layer 407 a.

The semiconductor layer 506 is a semiconductor layer of one conductivitytype (i.e., one of a p-type semiconductor layer or an n-typesemiconductor layer). As the semiconductor layer 506, a semiconductorlayer containing silicon can be used, for example.

The semiconductor layer 507 has a resistance higher than that of thesemiconductor layer 506. As the semiconductor layer 507, a semiconductorlayer containing silicon can be used, for example.

The semiconductor layer 508 is a semiconductor layer whose conductivitytype is different from that of the semiconductor layer 506 (i.e., theother of the p-type semiconductor layer and the n-type semiconductorlayer). As the semiconductor layer 508, a semiconductor layer containingsilicon can be used, for example.

As the insulating layer 509, a layer of an organic material such aspolyimide, acrylic, or benzocyclobutene can be used, for example.Alternatively, as the insulating layer 509, a layer of a low-dielectricconstant material (also referred to as a low-k material) can be used.

As the conductive layers 510 a to 510 c and the conductive layer 513,for example, it is possible to use a layer of a light-transmittingconductive material such as indium tin oxide, a metal oxide in whichzinc oxide is mixed in indium oxide (referred to as indium zinc oxide(IZO)), a conductive material in which silicon oxide (SiO₂) is mixed inindium oxide, organoindium, organotin, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, or indium tin oxide containing titaniumoxide.

Alternatively, the conductive layers 510 a to 510 c and the conductivelayer 513 can be formed using a conductive composition containing aconductive high molecule (also referred to as a conductive polymer). Aconductive layer formed using the conductive composition preferably hasa sheet resistance of 10000 ohms per square or less and a lighttransmittance of 70% or more at a wavelength of 550 nm. Furthermore, theresistivity of the conductive high molecule contained in the conductivecomposition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a π-electron conjugated conductivepolymer can be used. Examples of the π-electron conjugated conductivepolymer are polyaniline and a derivative thereof, polypyrrole and aderivative thereof, polythiophene and a derivative thereof, and acopolymer of two or more of aniline, pyrrole, and thiophene and aderivative thereof.

As the liquid crystal layer 514, a layer including TN liquid crystal,OCB liquid crystal, STN liquid crystal, VA liquid crystal, ECB liquidcrystal, GH liquid crystal, polymer dispersed liquid crystal, ordiscotic liquid crystal can be used, for example. Note that for theliquid crystal layer 514, it is preferable to use liquid crystal thattransmits light when a voltage applied to the conductive layers 510 cand 513 is 0 V.

As described with FIGS. 14A and 14B, FIGS. 15A and 15B, and FIGS. 16Aand 16B, the input-output device in this embodiment includes an activematrix substrate provided with a transistor, a pixel electrode, and aphotoelectric conversion element; a counter substrate; and a liquidcrystal layer including liquid crystal, placed between the active matrixsubstrate and the counter substrate. With the above structure, thedisplay circuit and the light detection circuit can be formed over onesubstrate through one process; thus, manufacturing costs can be reduced.

Embodiment 8

In this embodiment, electronic devices including the input-output devicein the above embodiment will be described.

Structural examples of electronic devices in this embodiment will bedescribed with reference to FIGS. 17A to 17F. FIGS. 17A to 17F eachillustrate an example of the structure of an electronic device in thisembodiment.

The electronic device illustrated in FIG. 17A is a personal digitalassistant. The personal digital assistant in FIG. 17A includes at leastan input-output unit 1001. In the personal digital assistant in FIG.17A, the input-output unit 1001 can be provided with an operation unit1002, for example. By using the input-output device in the aboveembodiment for the input-output unit 1001, operation of the personaldigital assistant or input of data to the personal digital assistant canbe performed with a finger or a pen, for example.

The electronic device illustrated in FIG. 17B is an information terminalwith an automotive navigation system, for example. The informationterminal in FIG. 17B includes an input-output unit 1101, operationbuttons 1102, and an external input terminal 1103. By using theinput-output device in the above embodiment for the input-output unit1101, operation of the information terminal or input of data to theinformation terminal can be performed with a finger or a pen, forexample.

The electronic device illustrated in FIG. 17C is a laptop personalcomputer. The laptop personal computer in FIG. 17C includes a housing1201, an input-output unit 1202, a speaker 1203, an LED lamp 1204, apointing device 1205, a connection terminal 1206, and a keyboard 1207.By using the input-output device in the above embodiment for theinput-output unit 1202, operation of the laptop personal computer orinput of data to the laptop personal computer can be performed with afinger or a pen, for example. Further, the input-output device in theabove embodiment may be used for the pointing device 1205.

The electronic device illustrated in FIG. 17D is a portable gamemachine. The portable game machine in FIG. 17D includes an input-outputunit 1301, an input-output unit 1302, a speaker 1303, a connectionterminal 1304, an LED lamp 1305, a microphone 1306, a memory mediumreading portion 1307, operation buttons 1308, and a sensor 1309. Byusing the input-output device in the above embodiment for theinput-output unit 1301 and/or the input-output unit 1302, operation ofthe portable game machine or input of data to the portable game machinecan be performed with a finger or a pen, for example.

The electronic device illustrated in FIG. 17E is an e-book reader. Thee-book reader in FIG. 17E includes at least a housing 1401, a housing1403, an input-output unit 1405, an input-output unit 1407, and a hinge1411.

The housing 1401 and the housing 1403 are connected by the hinge 1411.The e-book reader in FIG. 17E can be opened and closed with the hinge1411 as an axis. With such a structure, the e-book reader can be handledlike a paper book. The input-output unit 1405 and the input-output unit1407 are incorporated into the housing 1401 and the housing 1403,respectively. The input-output unit 1405 and the input-output unit 1407may display different images or may display one image, for example. Inthe case where different images are displayed on the input-output unit1405 and the input-output unit 1407, for example, text can be displayedon the input-output unit on the right side (the input-output unit 1405in FIG. 17E) and graphics can be displayed on the input-output unit onthe left side (the input-output unit 1407 in FIG. 17E).

In the e-book reader illustrated in FIG. 17E, the housing 1401 or thehousing 1403 may be provided with an operation unit. For example, thee-book reader in FIG. 17E can include a power button 1421, operationkeys 1423, and a speaker 1425. In the e-book reader in FIG. 17E, thepages of an image can be turned with the operation keys 1423.Furthermore, the input-output unit 1405 and/or the input-output unit1407 in the e-book reader in FIG. 17E may be provided with a keyboard, apointing device, or the like. An external connection terminal (e.g., anearphone terminal, a USB terminal, or a terminal connectable to an ACadapter or a variety of cables such as a USB cable), a memory mediumreading portion, or the like may be provided on the rear surface or theside surface of the housing 1401 and the housing 1403 of the e-bookreader in FIG. 17E. The e-book reader in FIG. 17E may have a dictionaryfunction.

By using the input-output device in the above embodiment for theinput-output unit 1405 and/or the input-output unit 1407, operation ofthe e-book reader or input of data to the e-book reader can be performedwith a finger or a pen, for example.

The electronic device illustrated in FIG. 17F is a display. The displayin FIG. 17F includes a housing 1501, an input-output unit 1502, aspeaker 1503, an LED lamp 1504, operation buttons 1505, a connectionterminal 1506, a sensor 1507, a microphone 1508, and a support base1509. By using the input-output device in the above embodiment for theinput-output unit 1502, operation of the display or input of data to thedisplay can be performed with a finger or a pen, for example.

As described with FIGS. 17A to 17F, the electronic device in thisembodiment includes an input-output unit including the input-outputdevice of the above embodiment. Such a structure can reduce adverseeffects of light in the environment where the input-output device ispositioned, so that the light detection accuracy of the input-outputunit can be increased.

This application is based on Japanese Patent Application serial No.2010-137080 filed with Japan Patent Office on Jun. 16, 2010, the entirecontents of which are hereby incorporated by reference.

1. An input-output device comprising: a first light unit including Zlight-emitting diodes, wherein Z is a natural number of 3 or more; asecond light unit including a white light-emitting diode and a lightguide plate on which light from the white light-emitting diode isincident; an X display circuit provided between the first light unit andthe second light unit, configured to be supplied with a displayselection signal and a display data signal in accordance with thedisplay selection signal, and configured to be set in a display statecorresponding to data of the display data signal, wherein X is a naturalnumber; and Y light detection circuits each provided between the firstlight unit and the second light unit, configured to be supplied with alight-detection control signal, and configured to generate datacorresponding to illuminance of incident light in accordance with thelight-detection control signal, wherein Y is a natural number of 2 ormore.
 2. The input-output device according to claim 1, wherein each ofthe Y light detection circuits includes: a photoelectric conversionelement including a first current terminal and a second currentterminal, wherein a current flows between the first current terminal andthe second current terminal in accordance with the illuminance of theincident light; a light-detection control transistor that is afield-effect transistor including a source and a drain, one of which iselectrically connected to the second current terminal of thephotoelectric conversion element, and a gate configured to be suppliedwith the light-detection control signal, and including an oxidesemiconductor layer in which a channel is formed and a carrierconcentration is lower than 1×10¹⁴/cm³; and an amplification transistorthat is a field-effect transistor including a gate electricallyconnected to the other of the source and the drain of thelight-detection control transistor, and a source and a drain from one ofwhich data corresponding to the illuminance of the incident light isconfigured to output as a data signal.
 3. The input-output deviceaccording to claim 2, wherein the photoelectric conversion element is aphotodiode.
 4. The input-output device according to claim 1, furthercomprising: a read circuit configured to read data corresponding to theilluminance of the incident light from the Y light detection circuits;and a data processing circuit configured to generate data of differencebetween two pieces of data corresponding to the illuminance of theincident light read from the read circuit.
 5. The input-output deviceaccording to claim 1, wherein the Z light-emitting diodes arelight-emitting diodes that emit light with a wavelength in a visiblelight region.
 6. The input-output device according to claim 1, whereinthe light from the white light-emitting diode is led to total reflectionin the light guide plate.
 7. The input-output device according to claim1, wherein the white light-emitting diode is configured to emit lightwhen the first light unit is not lit.
 8. An input-output devicecomprising: a first light unit including Z light-emitting diodes,wherein Z is a natural number of 3 or more; a second light unitincluding a white light-emitting diode and a light guide plate on whichlight from the white light-emitting diode is incident; an X displaycircuit provided between the first light unit and the second light unit,wherein X is a natural number; and Y light detection circuits eachprovided between the first light unit and the second light unit, whereinY is a natural number of 2 or more.
 9. The input-output device accordingto claim 8, wherein each of the Y light detection circuits includes: aphotoelectric conversion element; a light-detection control transistorthat is a field-effect transistor including an oxide semiconductor layerin which a channel is formed and a carrier concentration is lower than1×10¹⁴/cm³; and an amplification transistor.
 10. The input-output deviceaccording to claim 9, wherein the photoelectric conversion element is aphotodiode.
 11. The input-output device according to claim 8, furthercomprising: a read circuit configured to read data corresponding toilluminance of incident light from the Y light detection circuits; and adata processing circuit configured to generate data of differencebetween two pieces of data corresponding to the illuminance of theincident light read from the read circuit.
 12. The input-output deviceaccording to claim 8, wherein the Z light-emitting diodes arelight-emitting diodes that emit light with a wavelength in a visiblelight region.
 13. The input-output device according to claim 8, whereinthe light from the white light-emitting diode is led to total reflectionin the light guide plate.
 14. The input-output device according to claim8, wherein the white light-emitting diode is configured to emit lightwhen the first light unit is not lit.
 15. A method for driving aninput-output device comprising a first light unit including Zlight-emitting diodes, wherein Z is a natural number of 3 or more; asecond light unit including a white light-emitting diode and a lightguide plate on which light from the white light-emitting diode isincident; an X display circuit provided between the first light unit andthe second light unit, configured to be supplied with a displayselection signal and a display data signal in accordance with thedisplay selection signal, and configured to be set in a display statecorresponding to data of the display data signal, wherein X is a naturalnumber; and Y light detection circuits each provided between the firstlight unit and the second light unit, configured to be supplied with alight-detection control signal, and configured to generate datacorresponding to illuminance of incident light in accordance with thelight-detection control signal, wherein Y is a natural number of 2 ormore, the method comprising steps of: inputting the light-detectioncontrol signal to each of the Y light detection circuits; in a frameperiod set by the display selection signal, making the first light unitlit by sequentially switching the Z light-emitting diodes and emittinglight, and making the second light unit lit by making the whitelight-emitting diode emit light when the first light unit is not lit;and generating Y pieces of data corresponding to the illuminance of theincident light in a period when the second light unit is lit.
 16. Themethod for driving the input-output device according to claim 15,wherein each of the Y light detection circuits includes: a photoelectricconversion element; a light-detection control transistor that is afield-effect transistor including an oxide semiconductor layer in whicha channel is formed and a carrier concentration is lower than1×10¹⁴/cm³; and an amplification transistor.
 17. The method for drivingthe input-output device according to claim 16, wherein the photoelectricconversion element is a photodiode.
 18. The method for driving theinput-output device according to claim 15, further comprising steps of:reading data corresponding to the illuminance of the incident light fromthe Y light detection circuits; and generating data of differencebetween two pieces of data corresponding to the illuminance of theincident light.
 19. The method for driving the input-output deviceaccording to claim 15, wherein the Z light-emitting diodes arelight-emitting diodes that emit light with a wavelength in a visiblelight region.
 20. The method for driving the input-output deviceaccording to claim 18, wherein the light from the white light-emittingdiode is led to total reflection in the light guide plate.
 21. A methodfor driving an input-output device comprising a first light unitincluding Z light-emitting diodes, wherein Z is a natural number of 3 ormore; a second light unit including a white light-emitting diode and alight guide plate on which light from the white light-emitting diode isincident; an X display circuit provided between the first light unit andthe second light unit, configured to be supplied with a displayselection signal and a display data signal in accordance with thedisplay selection signal, and configured to be set in a display statecorresponding to data of the display data signal, wherein X is a naturalnumber; and Y light detection circuits each provided between the firstlight unit and the second light unit, configured to be supplied with alight-detection control signal, and configured to generate datacorresponding to illuminance of incident light in accordance with thelight-detection control signal, wherein Y is a natural number of 2 ormore, the method comprising steps of: inputting the light-detectioncontrol signal to each of the Y light detection circuits; in a frameperiod set by the display selection signal, making the first light unitlit by sequentially switching the Z light-emitting diodes and emittinglight, and making the second light unit lit by making the whitelight-emitting diode emit light when the first light unit is not lit;generating Y pieces of first data corresponding to first illuminance ofthe incident light in a first period when the second light unit is lit;generating Y pieces of second data corresponding to second illuminanceof the incident light in a second period when the first light unit andthe second light unit are not lit; and generating third data that isdata of difference between the first data and the second data.
 22. Themethod for driving the input-output device according to claim 21,wherein each of the Y light detection circuits includes: a photoelectricconversion element; a light-detection control transistor that is afield-effect transistor including an oxide semiconductor layer in whicha channel is formed and a carrier concentration is lower than1×10¹⁴/cm³; and an amplification transistor.
 23. The method for drivingthe input-output device according to claim 22, wherein the photoelectricconversion element is a photodiode.
 24. The method for driving theinput-output device according to claim 21, further comprising steps of:reading data corresponding to the illuminance of the incident light fromthe Y light detection circuits; and generating data of differencebetween two pieces of data corresponding to the illuminance of theincident light.
 25. The method for driving the input-output deviceaccording to claim 21, wherein the Z light-emitting diodes arelight-emitting diodes that emit light with a wavelength in a visiblelight region.
 26. The method for driving the input-output deviceaccording to claim 21, wherein the light from the white light-emittingdiode is led to total reflection in the light guide plate.